Steam Workshop: Preview Screenshot Squashed

Hey guys, just wondering if someone could help me with the steam workshop screenshots... I have put a park on the workshop and when I upload the wide F12 images I have taken in steam, they are squished in at the sides and are warped. I'm not sure what I'm doing wrong because other people don't seem to have the problem. Am I doing something wrong or do I have to change the size myself?
 
Doesn't this has to do something with the resolution / screen format (16:9, etc.)?
 
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