I have a full version of Acrobat, so I just Export the pages I want to bash to an image file, then work with the image. That is a fairly easy way to do it - as long as you have that capability with Image.
I currently use Paint Shop Pro, because its what I have, but its an older version, and I may look at GIMP or even suck it up and shell out for a better graphics program (hmm, or convince my company I need a newer one for our new site design). It works well enough, supports layers, and has lots of cool, nifty tools.
To be honest, though, the two most important things I need is 1) a way to be able to cut and paste in an image, and a way to do it with various sizes/shapes, and 2) a grid alignment tool. Rarely do I use any other effect - the rest is really just gloss (nice gloss, but gloss). If you can master just those two things (cutting and pasting, and using a grid to make sure your sizing is correct), you're 90% of the way to great kitbashing.
I will emphasize a point made earlier: whatever way you do it, save often, and save versions. You can always delete extra files later on.
As for Photoshop tutorials, look here: http://www.google.com/search?hl=en&q=photoshop+tutorial - google is your friend.
The Gimp will let you read in pdf files at whatever resolution you specified, so I'd been doing all my kitbashing at 300 dpi. 
