Would openGL be appropriate for building a 2d graphics app in the vein of photoshop?
I don’t think OpenGL would give you much.
Well it would give you all you expect. A canvas to draw on, ability to rotate and access pixels (sort of) etc. Obviously you would need to create all your own code for the various image processing algorthms that photoshop esque apps use. But that will be the same for any api
i wouldn’t take opengl for pixel oriented apps like photoshop 'cause opengl strength is its vector based abilities and this pixel stuff in opengl will be far more complex than using an pixel oriented api
if you need something similar to photoshop but cannot afford buying it try the gimp
if you miss some functions it’s plugin system lets you insert all what you heart can think of (or what your brain is able to code)
http://www.gimp.org/