I'm a bit of a noob, but hopefully not too much for this board - we all start somewhere

I'm working on a site that is basically template driven (sort of like a Myspace profile, where you only have a few fields you can edit, but the ability to use CSS in any of those fields).

My question: I know how to position with CSS using ABSOLUTE, but that doesn't work because even if I perfectly place an item (let's say 30px from the left, and 30px from the top), all the user has to do is resize their browser window and my image now resides in a different apparent area on the site... Possibly over text, possibly in a wrong column, etc. Quite an undesired effect.

Now with the template-esque site I'm working on, it's FULL of predetermined CSS IDs/Class'. There are probably about 100 of them. I can edit the CSS, but I have no idea how to really figure out what is what. It's either too much of a mess, or far from my ability of deciphering. So as far as I know, I can't fully take advantage of RELATIVE.

Does anyone have a way around this problem? How can I position in this situation?

You should be able to use relative positioning, but w/o seeing the template and the code you're talking about, it's impossible to give you any kind of solution.
Another possibility is to use floats, but even those may not be necessary. Newbies often start out using positioning on everything in an effort to make everything exactly pixel perfect. First lesson -- let go of that idea. Because of the way the different browsers render html and CSS, you will never get it to be EXACT. Save some hair pulling now and accept that.

Positioning everything simply isn't necessary. Learn to use the normal document flow to your advantage and then position elements only when needed.