Yeah, I typically do that from a computer. On the phone (using Chrome on Android) if I tap/hold the picture on imgur, then select “open image in new tab”, I can copy the link from the browser bar and paste that into the post editor “insert image” dialog. I find it best to remove the gibberish after the “.jpg” or (my preference) to change “.jpg” to “.png” and delete everything after. You should end up with something like: https://i.imgur.com/XXXXXXXXX.png
It is a little arduous, but the end result is clean on the forum and will scale with screen size. Don’t forget to set relative width or it will make phone viewing horrendous.