Try Gimp, its free. Its a huge improvement from Ms Paint, but a bit less of an improvement to PhotoShop. I find its the closest free thing you can get to PhotoShop and i personally like it just for creating logos and such.
Also, considering you want to use the mouse, I really recommend going into vector art. You can’t (not without a lot of practice) just draw a smooth curve with the mouse. Vectors and paths fix this, in that regard.. Inkscape.