The format of a picture file is the way that the digital data describing the pixels (dots of color) making up the picture is encoded in the file. Many different formats have been developed over the years, each one originating from a different early graphics program that came with its own proprietary method for storing image data. Formats are not interchangeable, and some have capabilities that others don't, because each one was developed independently by a different software company. Eventually, some programs came to be able to read and write the formats used by other, more popular programs, and the most popular formatsthe ones whose specifications were published for everyone to use in their own programsbecame standards on the Internet and form all the graphics we see in Web pages today.
Common formats are as follows:
JPEG - The Joint Photographic Experts Group format. JPEG files use 24 bits of data to describe each pixel, meaning that it can be any of 16.7 million colors. As its name suggests, JPEG is designed for images such as photographs and scanned artwork. JPEG files are the most common that are used on the Internet, because they can be efficiently compressed to very small sizes, which makes it easier and faster to download them on the Web. However, this compression is lossy, meaning that some detail data is thrown out and irretrievably lost when you save the file. Most software programs allow you to select a "quality" setting when you save a JPEG file; the higher the quality, the lower the compression, and the less data is lostbut the bigger the resulting file is. Most JPEG files look just fine when saved at a quality level of about 75%, which (for a picture 800 pixels wide) seldom results in a file larger than about 250 KB. Setting the quality level too low, though, can make sharp edges in the picture look blurry or blocky.
GIF - The Graphic Interchange Format, developed by CompuServe. GIF files store each pixel with 8 bits of data, meaning that each pixel can be one of 256 different colors, chosen from a palette in the software you use to create the file. There are several features in GIF files that make them still popular today even though their color range is limited: they can be transparent (having a certain color in the palette set to show the color of the image or text behind it in a Web page, and they can even be animated, incorporating a number of still frames with a certain amount of delay set on each frame. The fact that GIF files use a lossless form of compression means that they can very efficiently store all the data in a picture created in a program like Paint. However, GIF is a poor choice for photos or scanned art, or for rich digitally colored art, because the limit of 256 colors robs the pictures of their depth and realism.
PNG - The Portable Network Graphics format is a newer, wholly open standard that incorporates many of the best features of both GIF and JPEG. It can have 8 or 24 bits' worth of color; it supports true transparency and translucency with an "alpha" channel; it has lossless compression; and it can appear "progressively" on the screen as GIF and JPEG can both do.
BMP - The Windows Bitmap format is uncompressed. That means that even though it can fully support 24-bit color, it can take as much as 12 times as much disk space to store it, and 12 times as much time to download off the Web. For this reason, most Web browsers won't even display BMP images, for fear of encouraging people to use BMPs in their Web pages and making the Web that much slower. The TLKFAA does not allow BMP images.
The default format for pictures created in Paint is BMP. Because you can't use BMP files on the Web, you have to convert your pictures to a different format before they can be uploaded.
The ideal format to convert your Paint pictures to is PNG. This is because PNG uses lossless compression, so the sharp pixels and lines created in Paint will not be reduced in visual quality. PNG's 24-bit color support also means that you won't give up any color depth when converting from the original 24-bit BMP format.
Newer versions of Microsoft Paint allow you to save directly in PNG format, as well as GIF, JPEG, and others. If your version of Paint does not support these formats directly, use one of the programs found on the Software page to convert your pictures from BMP format to PNG.
If you don't have access to an image conversion program, or if you have a program that does not support PNG, you can use GIF as a second choice. GIF has lossless compression, so it won't degrade the quality of your sharp pixels or lines; and most pictures created in Paint don't use more than 256 colors, so GIF will work just fine for saving the picture looking exactly as it does in Paint.
Paint will save in GIF format, though it has a tendency to change the colors a little bit from what you had intended. For this reason, you should save in BMP format and then use a separate image conversion program to convert the files to GIF format properly. JPEG is a bad choice for saving the pictures you created in Paint. Because JPEG is designed for full-color pictures like photos, it achieves its small file sizes through lossy compression, and throws out "unimportant" image data, such as tiny variations between leaf patterns or the grain in the paper. If your pictures were created digitally in Paint, they don't have any of these details, and in their absence JPEG eats into the boundaries between color areas, such as right along the boundaries of sharp color lines. The result is a "blocky" or "muddy" look, and the loss of your pixel-by-pixel detail.
You can use PNG, the Portable Network Graphics format, in the pictures you upload. PNG images can be in 8 or 24 bits. with lossless compression and many modern features such as a proper "alpha" channel for translucent blending with background elements in Web pages (even though Internet Explorer does not yet support PNG translucency). PNG is supported by nearly all modern browsers and other software, and has nearly achieved the same level of de facto acceptance as a standard that GIF and JPEG have, though it's not 100% there yet.
PNG serves as a useful alternative for people who need the full 24-bit color support of JPEG but don't want their pictures to get mangled by JPEG compression. You have to be careful when using PNG, though: a 24-bit PNG image, even in modest dimensions, can have a file size ranging to 400 KB and more, which is about the file size you'd get if you'd just used JPEG at a very high quality setting. Be sure to experiment with different formats; if you can get almost the same visual quality in JPEG but save a lot of bytes in the file size, please use JPEG instead!