Only one cookie? Awww man. Me haz or else no help for you.
Joke.
Google "free banners" You'll find lots of websites that make free banners.
Problem is most have ads and some give HTML code witch Unreal Software doesn't support.
So it's really recommended to make your own signatures in software like Adobe PhotoShop and PhotoFiltre Studio X.
Paint - Basic (Anyone who's a notepad lover will be using this. Next to no features, but you really can't get lost in paint. Not unless you're dense.)
Paint.Net (Nice intermediary. Lots of features but not overtly complicated)
GIMP (Free Open Source Software alternative to heavy image manipulation tools like photoshop. Packed with features but quite complicated)
My advice is always to learn the hardest, that way, you will be able to use any of the others by extrapolation of knowledge. Gimp is really complicated at first, but there's plenty of tutorials and so forth, I've made plenty of transparency and animated images with gimp and it is thoroughly rewarding to have that much control over the final product.
PhotoFiltre Studio X, in former versions "PhotoFiltre Studio" is not free. You can get it in the illegal way or do like me: I didn't do in a illegal way, I just took the public key at the Brazilian community (please don't give me an illegal mark because the key was/is public and legal).
There's also the free version of PFSX, PhotoFiltre. Though, it has less functions/features. I don't recommend it.
PhotoFiltre Studio X GIMP PhotoFiltre Comparison, in my criteria...