Hi there,
It seams that the 3.10.7 rtmedia update fixed the issue. The pictures are now posted properly in the activity feed after validating the post. I encourage you to update rtmedia with this version. I was desesperate to find out how to solve this problem. I have disabled the "direct upload" option on my production website cause it was disabled also on my test website, but I'm not sure at all it has some influence on the problem. Have a nice day.