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.
![Tongue :P](./images/smilies/tongue.gif)
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.
![Happy :)](./images/smilies/cheerful1.gif)