After lots of trial and error I noticed that the dimesions vary depending on the size of your monitor. Mine still isn’t perfect but I learned that an 11:1 ratio was the closest to my desired fit.
Also, one option to upload could be to create a post, upload the photo via steemit, then copying and pasting the URL in settings. That somewhat worked for me, but let me know if it works!
Thanks @creativem1nd for taking the time to reply.
While you were typing I gave it one more try and it has worked. Instead of using the link from the post image page I copied the 'direct link' and that has worked.
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