What is the correct dimensions for an image in a video

Hey so when I try to upload image it sometimes gets stretched. So I tried 740*416 and other combos in height and width but it’s not working, what dimensions should I input in height and width fir an video so it loads properly?

You can use the following dimensions:

1920x1080 pixels (Full HD)
1280x720 pixels (HD)
854x480 pixels (Standard Definition)

I am not looking for thumbnails but inside the video. For example this kind of images what size should I use?

I don't exactly understand what's wrong with your videos? How are images stretching? Whether i used adobe premiere or Canva i never had a problem with images stretching. Only possible reason this could happen in my head is that when you started creating project/video you picked a resolution/dimension and then exported it into another resolution/dimension.