Unable to change header image

Hi, newbie to this forum.

Since the auto upgrade to 5.3 I’ve had no end of problems. Constant errors every time I upload an image and when I try to post anything. It’s been horrendous!

But I’ll get onto them in another thread later. Right now my pressing issue is I’m unable to amand my site header image. I select the image I wish to use, but when presing the “Select and Crop” button, absolutely nothing happens. The image is usually visible for editing purposes but it’s completely blank. This may be related to the issue I have uploading new images as the same thing happens.

Apologies if this has already been covered elsewhere but I cannot see it. I have a screenshot but can’t see how to upload it.

The page I need help with: [log in to see the link]