Why you do not use the option take a snap that is already provided on dtube. It's easy, take a snap of the favorite part of the video and download it. You can edit the picture you just snap from the video and then upload it in the same format size. 
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I've tried that and it doesn't work for me either. If I click it I go through to a screen with just that image and if I click back I go back to square one and have to upload my video and write my description etc again. It could be because I'm using safari and dtube was built to work with Windows rather than Mac
Am actually using dtude with FireFox. I got same problem with safari and chrome. But with FireFox it works fine for me.