Inserting image into pdf

I am having a very vexing problem that could be a showstopper for me. I am editing an art manuscript in pdf form, and I need to insert images. I use the "Insert Image" command, and it dutifully lets me choose the image, lets me move it around the page, but it simply won't let me insert it. It won't drop it. Perhaps I am missing something simple. I click. Control-click. Option-click. Double click. Nothing.

I am using Acrobat Pro DC on a Mac running El Capitan.

Any ideas?

Thanks!