lol do you know how powerful the DS is? You are never going to run anything NEAR photoshop-like image editors on it. My laptop is far more powerful than a DS and struggles a little with Paint.NET (a slightly less advanced, but free, image editor than Photoshop).
The best you are gonna get is probably UAPaint. There's also Colors! and Phidias, but they're more for drawing stuff than editing. Colors! is pretty cool though. The harder you lean, the thicker the "paint" is applied.