Small image ontop of texture

I have created a (rather beautiful) barrel in blender with a rusty metal texture. I now wish to add a small radioactive image on the side of the barrel on top of the rust texture. When i try to include an image into uv /image editor it expands the image to take up the whole screen, is there a way to rescale this image and place it on the desired part of the barrel without affecting the rust texture for the rest o the barrel?Thank you.

Go to blenderguru.com, there is a tutorial showing you how to do this with steel barrels.