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[Yak]

Mostly they'll be .png or .gif files, made using Photoshop or a similar digital art program.

If you don't have any, GIMP is a good open source image editor which can be downloaded from here.

There's loads of good tutorials out there on the web to help you get started if you just google around a bit, and you'll probably get an answer if you post any specific questions on the forums here (assuming someone here knows the answer )