Photo programs such as photoimpression resizes photos for you, if you don't have that then you could email the pictures to someone on here who does have a program that resizes photos for you.
The banners in our signature with our pets on them are made from either one of us or someone else made them. Mine was made by Fina. If you want a banner like ours, you can post photos of your dog(s) under the banner request topic in general/miscelaneous.