I would also recommend that you get yourself a dummies guide to whichever photoshop version you have and learn how to use it..it is the most powerful image editor and well worth the learning curve ..and there are also free tutorials for it all over the web..
Gimp is free but not very easy and intuitively to use.
I never liked Photoshop - it's very powerful but the average webmaster does only need a fraction of the functions. Too bloated for my liking. It's like flying a space shuttle when all you have is a drivers license for a car.
I have been using Fireworks for years now and I think it's still the best tool for the average webmaster. It's not free but much cheaper than Photoshop and in my opinion much easier to use. I only live in constant fear that Adobe will abandon it one day, since they bought Fireworks from Macromedia a few years ago.
Here a good website comparing Fireworks and Photoshop. 50 reasons not to use photoshop for webdesign:
IrfanView is also free and supports most Photoshop plug-ins. It comes per-configured with many free ones.
I know of a few local universities that use it for their image editing classes.
I've also used it on websites I've taken over that had all their thumbnails saved at 100% quality. That was using it's batch mode capability. Saved me a LOT of extra clicks! And of course saved him lots of bandwidth too :-)