You can certainly output a still from Premiere Elements. You can even custom-create it to a specific resolution. (I'm assuming you know how to then use this still as your thumbnail on YouTube.)
Do you need this still photo to be at a size other than 1920x1080?
Hi, thank you. Youtube requires 1280 x 720. I have to take time to play with this as I've never tried to create a thumbnail from a video still. Thanks!
Go to Export & Share/Image -- then select the 1280x720 resolution and the JPEG output option. Click Save.