Spammers commonly conceal controversial "hot button" content in images, making spam filters suspicious of large images or high image-to-text ratios. Emails consisting entirely of one large image are filtered by default by many spam filters.
A large number of images also increases the kilobyte load of your email, and some ISPs may filter or throttle your emails to conserve their bandwidth. Optimize your images to keep them under 50 KB each.
If you want to deliver a large number of pictures to a recipient, link them back to a slideshow on your website.