You can't limit the number of images, but you can limit the overall size of the signature and the image sizes.
Try adding this to EXTRA.css:
Code:
.message .signature .bbCodeImage {
max-height: 100px;
max-width: 200px;
overflow-x: auto !important;
overflow-y: auto !important;
}
You can change that code to suit, adjusting the sizes and the overflow type (hidden for example).