You can choose custom thumbnails for your videos. You might want to do this if the randomly generated thumbnails aren't entirely representative of the content you're uploading. Go to 'Content' and then 'My Content' in the sidebar - you'll be taken to a page where you can see all of the videos you've uploaded. If you have a lot and you're having trouble finding the one you want to edit, you can sort by category, video status, or filter by category or tag.
When you find the video you want to edit the thumbnail for, click the thumbnail it already has, then click the 'Thumbnail' tab in the pop-up.
From there, you can click the 'Upload Custom Thumbnail' button and search your computer for the thumbnail you want to use. You can also upload multiples, so you can pick your favorite from a list.
When you've uploaded a thumbnail (or multiple thumbnails), select the one you want to use. It will highlight purple around the edge to indicate that's the one you've picked. You can't select multiple.
When you're done, save your changes.