Thanks for getting this discussion going.
Its a subject that always fascinates me, as there are so many ways to get inspired. One of my favorite ones is creating a texture or pattern background, then playing with different effects and brushes, just being totally spontaneous, with no attachment to the outcome. The results of such a process will usually be abstract, but sometimes a form will emerge or I will get an idea for another painting, and I go on from there.
Sometimes I will scan a leaf or a flat flower and start with that form or color.
I also have a lot of graphite and pen & ink drawings from my analog days that can be scanned, then colored, or altered in some way digitally. I've really enjoyed simulating graphite and pen & ink effects with painter.
Often music will start the creative energies flowing, and once a work is started, will help keep me at the computer. For instance, two of my dragon paintings were inspired by the Vangelis piece titled, simply, "Dragon" from his China album. For some reason, electronic music is more effective for me than classical or rock, perhaps because it is technologically created.
I have a growing art book library, but try not to go to other artist's work for my ideas. However, my wife gave me the ultimate art book for Christmas, titled "30,000 Years of Art", published by Phaidon. It has proved very stimulating and endlessly fascinating. I love to create "ancient" mythological art with modern technology.