Ahhh, yes, that would do it. When adding an entry or adding an image to an existing entry, that WP error would have bubbled up and displayed in the admin as an error.
One of WP secrets is that is actually requires GD too but never tells the user. Without it it is unable to create its own thumbnails and such for featured images. WP instead silently servers the full hires image using CSS scaling. A great way to slow down ones site :/
Glad to hear you figured it out!