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Philip has asked some good questions (thanks Philip).
Apart from that I have checked your Yola account and created a test page called "Test" to test the banner upload functionality (don't worry, the test page is not in your navigation and you can delete it at any time yourself).
It worked fine for me, and I was able to upload a banner image quickly (which you can keep and use if you want).
I've also noticed that you seem to have managed to add a custom banner image. Is this the image you want or did you want to change it and replace it by a new image?
Remember that your banner image shouldn't be too large (20-150KB) and have one of the most common file extensions, such as jpg., png., .gif, etc.