From a mobile click choose file
Then files or whatever it's called on your phone
Then select the picture u want from an album and click upload pictures
Ive done two as an example so you can see one that uploads and one that doesn't.
If it works you'll see the picture and if it fails you'll get an error message so you'll need to resize it.
My rule is if it's over 1mb it won't upload. I'll let someone else explain horizontal and vertical axis's as it's too much for my brain to comprehend