Adobe is the monopoly in the desktop graphics tool market. Not sure why no VC targets it at this moment, which does seem like a slow dinosaur to attack on.
Pixelmator's proprietary, and can't run on non OS X ATM due to reliance on, IIRC, CoreGraphics or similar. If they recreated those libraries on other platforms they may have a PS competitor.
Any startup that sets out to recreate Photoshop would also compete with GIMP and Pixelmator.