Thanks so much for the feedback everyone. I've just learned that a JPG is already compressed so I'd just need enlarge my 1.4 MB JPGs a little over twice to meet the 3MB submission threshold and give submitting through Alamy a shot. Stockimo sounds like a solid second option though.
Just out of curiosity, is there any way to double the size of a JPG without using Photoshop, i.e., any online enlargers anyone knows of?
Thanks again!