joliett89 - I'm tending to agree with tangor on this. I too have an extensive art, graphics and design background, but, more often than not, I pull in a professional graphic artist whenever I can. tangor's comment that "sometimes we can't be everybody else" covers quite a bit. ;)
That said, there are reasons to give it a try yourself. My background in the field helps me to be a much better communicator with the artist than I otherwise would be, and much better at conceptualizing what I want... and I realize that not all projects have budgets for outside help. Additionally, creating visuals can be a great pleasure.
So, if you're going to give it a shot... and you're right, practice is a big factor ... let me say at the outset that familiarity with the software, while an accomplishment in itself, is not going to give you the design fundamentals you need to make even a dent in what's a very deep and complex craft. It helps to learn those basics.
For the basics in layout and typographic design, I suggest "The Non-Designer's Design Book"
by Robin Williams. It's subtitled "Design and Typographic Principles For The Visual Novice
", and it's the best such book I know. Williams has also written a web design book that I don't like nearly as much as this one, and in fact I don't recommend her web design book at all.
I would also spend a lot of time looking at art and graphics. In terms of web graphics for the kind of utility work you're talking about, Smashing Magazine [smashingmagazine.com
...] is an online standard which I think I can safely link to. It also gets into tools and CSS.
This forum has an extensive library of posts that you definitely should check out. I did some searching using our site search to see what might be useful here.
The first search I tried... creating images for the web site:webmasterworld.com
...returned this thread as the first result.... Graphic design & creating web graphics from scratch?
It's an excellent discussion, and should provide some more food for thought to get you started. As you'll see, there's no one easy answer, and there are a lot of aspects to the field.