I am having the exact same problem. Although my understanding is that the precise image header size is 740 x 171. But it should adjust the size in the header space.
It does seem to be some kind of cache problem, but I too have tried deleting cache. I’ve even trashed the original I made but it still shows up instead of the new modified one I want on there, and the one that shows up in my dashboard. Says it is successfully uploaded.
Tried again and again but to no avail. Please help.
