This is a brief explanation of how people misuse the JPEG (aka JPG) file format for screen-captures, pie charts and any image with "simple" colors.
JPEG stands for Joint Photographic Experts Group and GIF stands for Graphics Interchange Format. GIF is technically outmoded and some apps won't save GIFs due to legal issues, but it remains popular. As one of the first widespread image formats (1987) it also has nostalgic value.
If you see a mottled look in a screencap, someone probably mangled basic hues with JPEG 8x8 block processing; unaware of the technology behind it. Such images end up looking worse and occupy a lot more disk space than they ought to.
A pure black and white image saved as a JPEG is very wasteful, but people do it often with cartoons on the Web. Aside from having an ink-bleed look, these messy files also slow down page loading.
Bitmap (BMP) images, which reproduce every pixel without compression, are notorious for wasting bandwidth in emails. Another culprit is failure to resample (shrink) images to a size appropriate for email or a website. Think twice about using an original 3mb photo when a scaled down 300kb version is fine.
The general public seems naive about matching file formats to actual image content. That's OK on a small scale, but their apathy is probably slowing down the entire Internet!
Various other algorithms like PNG (touted successor to GIF) and the venerable TIFF are not discussed here.
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