Graphics and images play a major part in sim-games. Especially horse ones. Use these tips to make your picture decisions:
Personal is better - if you can scan in some of your photos or use a digital camera. It will be unique and one of a kind on the web
Make sure they are a appropriate size - The smaller they are the quicker they load
If they are big and you can't scale them down or crop them, or you don't want to thumbnails and links to the bigger image work just as well
Picture clarity - Nothing I hate more than an unclear picture. If it's not a good quality picture, don't use it
Make your own graphics - Get a program (from any electrical shop - Photoshop, Print It!, a cheap one) and make your own. It is so personal, and shows so much effort. Add your own captions. See here for banner tips.
Most importantly have fun with graphics!