1) You're posting in the completely wrong forum. You're new, so I forgive you.
2) Try searching! We have many tutorials here at the Hive, and I know for a fact that the question you present is one of the most commonly asked, it's covered somewhere.
I'll answer your question anyway:
Warcraft III uses .tga files. Paint Shop Pro (free) and Photoshop (pay) are two very well known pieces of software that can save in this format.
But, you don't need either of them.
What I'd recommend is that you search the Tools section for a little program called WC3 Image Extractor II. This let's you convert the image to the appropriate size and format. It's very easy to use.
In terms of finding actual images? Most people resort to two of three options: Making a standard text and plain-background image themselves, or Google. The third option is having someone design the art professionally for you, but this is costly, so only very serious mapmakers that have a big project tend to go this route (DotA and Element TD are examples of such).
