Can I mix the kinds of images I use on a web page? Suppose I wanted a GIF background, a couple of JPEGs, and the head title done as a PNG 24. Will the combination automatically crash the browser when someone tries to open the page?
It shouldn't. There's no reason not to mix image types, provided that there is a reason for saving the files that way in the first place. If you have an image with only black and white, why save all those colors that aren't included? Make it a 2-bit GIF and enjoy the savings in file size. Photos need to be saved as JPEGs so they look better onscreen. But sure, so ahead and mix the image formats you use on your web pages.
What's the deal about 216-color GIFs? Are there different kinds of GIFs for Macs and Windows computers?
It used to be that the Mac and the PC had an 8-bit color mode, which meant that they could display 256 colors. But, just to be difficult, the people who designed the two systems used slightly different colors. Only 216 of the 256 colors were close enough to be considered the same color on either screen. So, to be completely compatible, to make sure that what you saw was exactly what everyone else saw, we used 216 colors as the limit for a GIF. Now, monitor displays have improved and most computer graphics users agree that "close enough is good enough" and use the entire 256-color table.
Are there any advantages to lossy compression, and if not, why isn't there a better way?
Lossy compression simply means that you lose some data when you save the picture. Groups of pixels are merged into a single value, and only that value is saved. The first time you save a photo as a JPEG, you suffer some loss, but so slight as to be unnoticeable. The second time you save the same file, more data is lost. Think about the numbers. Suppose, in the first save, you had groups of 9 pixels (3 by 3) that were averaged down to 1 pixel color for all 9. In the second save, pixels were again divided into groups of 9 and averaged. But this time, each of the 9 is already the average of 3 pixels, so you now have 27 by 27 pixels or 729 total, averaged into groups of 9 and reduced to 1. At this point, you have visible loss of data. Further saves make it worse. Lossy compression isn't a problem if you save as a JPEG only oncein fact, it's a very efficient system. The problem builds as you repeat the save. Why isn't there a better way? Many users think PNG is better. I'm not convinced, but with the entire Internet and everything that goes on it in a constant state of flux, we shouldn't have long to wait for the ideal solution.