From time to time we get reports from people posting from mobile devices that they are getting an 'invalid dimensions' error when they try to upload an image.
We have found through testing that if you carry out an edit of the file and save it, you should then be able to upload the image without a problem. It could be as simple as applying the auto adjust tool in your device's photo editor.
It doesn't happen from time to time, with the iPhone SE third generation it happens every time. I shouldn't have to edit a photo or choose the basic upload just to post a picture. Really frustrating. Safari browser - standard settings