I want to know because for my run I don't need video and screenshot takes less time to do, but I don't know how to put the image I want to use in the description. Can someone tell me how?

you can upload your image to gyazo or imgur or whatever you'd like to use and then when you submit your run copy and paste the link to the image into the description

You can also put an image link in the "video link" field if you want to and it will embed the same as if it was in the comments.

Downside to this is it shows up as having a video which can be annoying for someone else looking over the run who thinks they're going to get a video but end up with just an image.