I just found two very simple but helpful things! I’m so excited I had to share.
First, a nice list of specs for various popular video upload sites:
and second, a dummy image generator:
(All you have to do is modify the URL to generate placeholder images of any size: http://dummyimage.com/390×321)
Both of these sites would have come in handy a zillion times over the last few years. Give them a try!