I get an "HTTP Error" when attempting to upload images.

I get an "HTTP Error" when attempting to upload images. This happened suddenly on 5 different Wordpress installations, all different themes. I called my host (inmotionhosting.com) and they claimed it must be the theme, a plugin or WP install - but this is happening on all sites, different themes and was working just fine on all sites. It only happens with images over 100k. I changed nothing on the server. I'm baffled!