I don't understand the image score, and I think I would like it if someone could explain it. It looks like I can't vote pictures up or down to change their score, and I'm not sure if the score is in any way related to the picture's favorite count.
If you are Gold member or higher: You can choose to vote each post up or down (or not vote at all, of course). When you favorite a post it will automatically receive +1 score (as well as +1 favorite count). Score from favoriting and score from voting are separate, so you can give an image +2 score by both voting it up and favoriting it.
If you are a basic member: You can't affect post score at all. Favoriting a post with increase its favorite count, but not its score.