Here's a quick tutorial on how to render a character on different background in MS Paint:
Step1 - Here are the images I want to use. Balto and the background with wolves.
Step2 - Use free hand selection to cut Balto out of his original background. Doesn't have to be perfect. My character has smaller ears than Balto so I didn't select them perfectly.
Step3 - Drag your Balto to other background.
Step4 - Using the same selection tool clear the image. Just select some parts and click Delete button. Remember to save some lines for further reference.
Step5 - Here's how it looks now. Still not so pretty, eh?
Step6 - Now it's time to lineart it! Choose pencil tool and just lineart over the image
Step7 - And after touches of smaller pencil, it's done. Now it's time to colour.
Step8 - I lineart the markings first. Notice that linearted lines are in slighty darker tone than actual colours of the character.
Step9 - I drew scar and eyes in the meantime. All coloured and done!
At the end, blurred in IrfanView:
