Bill is probably correct about the size of smaller graphics - mught not make much difference whether they are jpeg or gif. I may do some 'scientific' tests later...
I don't think ColorIt has that kind of batch processing, but I am now excited about Elements 2 (just got a couple of books on it, looks extremely interesting). It does have a batch function that can do this. I think Graphiconverter also has this capability. Never used PS.
CSS is a little less intuitive than HTML, but it is not rocket science either. However, it is less well supported by various browsers and can cause differences in what viewers see, unless you limit what parts of it you use/limit the browsers you support/use a few 'hacks' for specific browsers. As an example, sneakers sent me a screen shot of what our church site lokks like in Opera/OS X. Pretty bad! No one had mentioned the problem in nearly 9 months of the site being live, but that is mainly because there are so few viewing it and I doubt more than 1 or 2 have even herad of Opera! And ultimately, it is my fault for not testing properly.
Get a good book on CSS. Eric Meyer is a great writer and a CSS expert. "CSS, The Definitive Guide," O'Reilly.
Jim C.