Comment 1 for bug 1084692

I hit the same problem. I did a google search and came upon this bug report. I also have a workaround. For the next person who sees the problem, googles, and ends up on this page, here's that workaround:

file > Export video > Advanced > Advanced Options

In the Video & Audio selection menu, choose Image Sequence.

I'm not sure if this next step is strictly necessary, but I did it anyway. Click Profile, and make sure the selection menu matches the type of Image Sequence you want. My video is 1920x1080, so I chose HD 1080p 60 fps.

There's only one Image Sequence Setting available, png.

Look back up to the top of that dialog box. Double check that the "Export to Folder" and File Name are what you want. You are about to make a bunch of files in that folder. Click Export, and wait a while. Openshot then creates still pictures in your desired resolution.

Be aware also that the numbers in the files OpenShot creates are not padded with leading zeros. So, for example, the following files will appear in a listing of the folder, next to each other:

still_from_video_999.png
still_from_video_99.png
still_from_video_9.png

even though they are from different parts of the video. I think I'll submit that as a separate bug.