Instead of giving us limited options and forcing us to select thumbnail manually on each site, or not giving us an option at all, every agency should recognize the poster time / frame and set it as the thumbnail on their sites automatically. This would make it easier for both sides and it would potentially increase sales (we all know how much the adequate thumbnail means for a video).
For anybody who doesn't know, you can set the poster frame / time in After Effects (composition > set poster time) and in Quicktime pro (view > set poster frame).