This is an old bug that's regressed but I can't find the original.
Here's the bug:
- Open a profile with 4 images in it already. Edit it.
- Remove one image. The button for adding a new image doesn't appear.
- Remove a second image. Now it does.
Alternatively you can do step 2, then save your profile, then refresh the page, then go back to editing it, and the button will be there.
The button should appear immediately after removing one of the images.