I have not received any messages when I recently uploaded my clips that I always save as Photo-JPG .mov

If your raw footage was recorded progressive (and NOT interlaced) than Photo-JPG is perfectly fine.

Only if you have interlaced footage (older 4:3 aspect ratio stuff or camcorders) only then I would still use the Motion-JPG compressor.

Did Pond5 announce anything lately in that regards?

From Pond5: "Thanks for the upload. While we are not yet disallowing Motion Jpeg codec, we are encouraging contributors to use ProRes422 codec since it is higher quality. "

What should I be using for "higher quality"?

Or do I need to open these in Elements and then "Save As" something else. If so, what's the best format to save my files in for Pond5, possibly SS and IS also. Is there one that works for all of them?

I do have the same problem with English as you have. I'm using H.264 almost without exceptions. Trying to get files uploaded to Shutterstock, I've tried other codecs, though. Having said that, all my efforts in trying to find the right codec for SS where back when I used Final Cut Pro. I've now bought Final Cut X, and all of a sudden, all sites just swallow the files I produce. I still don't understand what's going on. I'm still compressing using H.264, and the settings seem to be identical.

Somewhere, back in my mind, I remember reading there were some potential intellectual property issues with using H.264, commercially. Sean had something intelligent to say about that, if I remember correctly. Can't find the thread, though.

It's also a matter of desinterest, and a general lack of understanding, that I stopped caring. It didn't work then, but it works now, so I'm happy. But down below, it keeps nagging me about what the difference is between the vast amount of options and how these codecs work.

We're going to the IBC trade fair this saturday (by Pond5 invite), to hopefully find some answers, or at least to find a proper direction towards those answers. If anyone has a question for me to ask, I'll be happy to oblige, if I have a chance.