I use Photoshop now, but before that I used
GIMP. Personally, if you want to try out making graphics, that's a great one to use. It's free and fairly easy to learn the basics.
As to your question about blending pictures together, well, it varies.
When pictures are from the same scene, it's very easy to make them fit together - they already are colored much the same. When they're from completely different scenes, I tend to use lots of gradients or textures to just change the color up altogether.
If you want to try out GIMP sometime, I've got a tutorial of the very, very basics up
>>here.
If you've got more specific questions, like showing a made fanart and asking how it was done, I'd be happy to answer things.