I'm a hard-core Photoshop user. I use it nearly every day. All of the graphics for the upcoming Solarian skin were created with Photoshop. It's not a good pixel painter, though, unless you're very familiar with it.
I don't know of any such tutorials - certainly not Trill related - but deciding exactly what you want before you start and taking a look at others' work in the same area is a good place to work from.