I was changing the thumbnail dimensions in the rtMedia Settings in the wp-admin area and they do not make any changes in how images are cropped. I set the thumbnail sizes, uploaded a new image to my account on the front-end (in my profile), and then took a look in my FTP. I saw these sizes:
image-1024x512.jpg image-320x180.jpg image-840x300.jpg image.jpg (this is the original)
840x300 is the only one that reflects a dimension that I put in (this is the medium-sized image), but that was a prior thumbnail size that I used. The ones that I input before uploading the latest image was 200x200 for thumbnails, 0x0 medium, 0x0 large. (I wanted to test disabling thumbnail creation by setting 0 in the sizes, as Wordpress itself allows this).
The featured image size crops perfectly when opting to set a Featured Image in my profile on the front end.
-- This is using the latest WP, BuddyPress and bbPress, if that's a factor. Fresh install of rtMedia, somewhat fresh installs of WP/BP/bb.
Thanks for looking into this, and especially thank-you for this awesome plugin. There have been some problems, but seeing as how this is free, I can't complain -- I can only try and help by reporting any bugs.