You should use external software to make your picture square. The maximum pixel size is 512x512, anything larger will be condensed.
I use Paint3D and select the 1:1 ratio to make my pictures square.
When you’re satisfied save as another picture. Simple. It’s not that difficult. You don’t need to mess with Roblox Studio Screenshots. I use the Snipping Tool to take my screenshots. That’s a hard installed application on Windows.
Also wanted to mention that thumbnails have a ratio of 16:9 and are at maximum size, 1920x1080. Anything above that number will be condensed. Just thought this was related.
In my experience, screenshots from inside roblox studio usually don’t cut it. Editing your own photo or a screenshot will make the icon more appealing to someone who may see it while scrolling. Also, icons are square, measuring at 512x512 pixels, or a 1:1 ratio. If the icon in the provided picture is what you want to edit. If you want to edit the wider picture next to the play button, those are measured at 1920x1080 pixels, or a ratio of 16:9.