Also, I'm wondering if I can use the "save for web" feature in Photoshop, which compresses the file-size somewhat (I think - I don't really know much about compression, here). I like it because it lets you see how big the file will be, saved out as different types. But will that compression adversely affect the image quality (significantly)?
Not particularly familiar with photoshop itself. But yes, chances are it will be trying to save as jpeg, and for web it will be increasing the (lossy) compression to reduce file size, because it's not so noticable on the web.
Also, now that I'm thinking about it, how do you make a link so it just downloads an image, rather than attempting to take you to a webpage which displays it?
Don't. Users should know to do whatever they need to do to save the image. If they don't you can tell them (right click -> save as for windows, Ctrl-click -> save image... for mac is the usual, but it is actually browser specific, so unless you want the mess of detecting browsers and trying to provide information based on that (and noting that some browsers lie in order to be compatible with pages that claim to require a certain browser), I really won't bother) Hope this helps some. Timothy