Adobe Photoshop - For a fee
The most used for raster images, useful in some cases as well for vector edition, but it's not as versatile as Illustrator.
Painter - For a fee
Not as popular as Photoshop, but its really powerful for raster images creation. Oil painting could be perfectly simulated with this program.
PicMonkey - Free
This online image editor counts on dozens of tools, filters and effects so as to correct, improve, decorate and customize your pictures.
Gimp - Free
With this software bitmap digital images may be edited, photographies as well as drawings. It's quite similar to Photoshop.
Krita - Free
Has similar features to Photoshop, but it's oriented to drawing.
Picadilo - Free
Two tools are offered in this website: a quite complete image editor and a very simple tool that allows you to compose collages directly on your browser.