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.
Photoshop may be the industry standard, but for skinning and general web gfx, I wouldnt go past Paintshop Pro...and most PS plugins work with it very nicely. Also its affordable.
for tutorials in gfx stuff using psp, you cant go past Pinoy 7