Welcome to the forum and your first post! And it sounds like a frustrating problem. Realize most of us are just people like yourself trying to help...
I don't know, but I would suspect your file size is too large, either "per picture" or the total size of the attachment if attaching multiple pictures. Remember computer screen resolution can be much smaller for a photo compared to what you need to make a photographic print, without noticeable quality loss. And your missionary has a limited time to use the computer, so sending a really large photograph is a waste, especially if they "bounce."
Your camera usually takes photographs in a resolution for making prints. It may have a "web" or "ebay" or "email" setting which would give much smaller file sizes for computer display. However, I suggest keeping the files at "print" sizes when you take them in case you want prints (and enlargements!) and then "shrinking" copies of those same photos afterward, that you can then send to your son.
I just asked our missionary, and he he gets ours at his ldsmail address, and I've never had any "bouncing" photos. What I do:
I don't send a lot at once,
I always make them much smaller than they were originally from the camera. I use photo editing software to reduce the size of the file. (72 dpi is screen resolution, so I set my photos to 72dpi with length and width around 4 inches or so.) Your software might have a "web" size.
I use JPG format (it's compressed) and set the quality to mid-range or lower when I save it to my computer from the photo editing software, (after editing them to reduce file size)
I usually put them "inline" (using the attachment feature) in the email
There are many "free" photo editing software packages around, some online, and many cameras and scanners ship with software that will resize your photographs for sending to your missionary. Remember to keep the fullsize originals, and make a copy of the smaller files for sending.