Unable to Replace Preview Image

[hawaiiski]

"After adding the video to your site, just navigate to it and then click Tools > This Photo > More > Replace Video Preview Image. Be sure to choose an image from your computer that matches the dimensions of your video."

On Elements 8.0 I exported an image & customized it to the same dimensions as my video, but the attempt failed. The SmugMug team said the image is the wrong size. How exactly do I export an image to match the dimensions of my video (1280 x 720)?

[Andy]

Crop it in Elements to EXACTLY 1280x720. Then try.

[hawaiiski]

I'm not seeing dimension sizes when I crop it. There is an area at the top where I can type in height & width, but it's for in., not p. Shouldn't I be able to do this when I click File> Export as New File(s)?