Two image formats have become the standard for displaying images on the web: GIF and JPEG. Both formats use compression, which is a very good thing, because they take up less disk space and can be downloaded much faster than uncompressed pictures. (Try converting between one of these formats and e.g. BMP and look at the file sizes.) JPEG images use an algorithm that mean the images are actually altered during compression, as information is lost. The person who makes the images can decide how much compression he/she wants, which is a good thing, as too much compression makes the image look bad. GIF images, on the other hand, keep all the original information. GIF images are limited to 256 colours.
All this means that GIF and JPEG are suited for different kinds of pictures. JPEG images are used for photos, while GIF images are used for pictures that have well separated areas in different colours, like C-64 screen shots. (Try converting one of those to JPEG format and see what happens.) So most C-64 sites that have screen shots use GIF images. Then why don't I do that?
The GIF compression algorithm is copyrighted, and it is not clear whether it's allowed for free programs to use this algorithm. Therefore writers of software that creates GIF images will have to either get a license to use it or hope that they will not be sued. Or they can create fake GIFs, that aren't compressed. None of these solutions are good, for different reasons. For developers of free software it's not an option to use a non-free algorithm. And fake GIFs get very large, so that's not a very good alternative either.
I only use free software, because I believe that software should be free (think free as in free speech, not free beer). Proprietary algorithms are bad things, and should not be used. If you want to know more about free software, take a look at The Free Software Foundation. They also have more information about GIFs, which might be of interest to you.
The image format I'm using on these pages is an open format, called PNG. It's a superior format to GIF, both technically and license-wise. I'm not sure if all browsers support this format, but I know that Netscape 4.x does. If your browser doesn't support the PNG support, you're out of luck. If you really want to see these images, you will have to download them and view them with some image viewer. Some free programs that can display PNG images (and lots of other formats) are The Gimp, ElectricEyes, zgv and ImageMagick.
If you think this is a problem, you shouldn't blame me, because I'm not responsible for this. Blame it on Unisys and IBM who patented the LZW compression algorithm, and more generally everyone who supports software patents. Of course, you may ask why I would care about this, when everybody use GIFs anyway. Well, if no one ever says no to crap like proprietary file formats, nothing will ever happen. And I hope that this page can make at least some people think about it.
Hopefully the PNG format will soon be in use all over the web, so that there won't be a GIF problem anymore.