If you've tried to upload an image and you're seeing an error or the image doesn't show up, there's a couple of reasons why this might be happening.
The rule of any image for online use is keeping the file size as small as possible while retaining a decent image quality. In the best possible scenario, keeping images under 1mb would be ideal, 1-5mb isn't a problem but depending on your connection speed, this could start to cause problems the larger you go.
If you do see an error or issue with your image, check the file size first and make sure you're not using an image that's way bigger than it needs to be. If it is, compress the size to as close to 1mb as possible and try again. If you don't have a means of doing this, there's plenty of free online image compressors that'll do it in seconds for you.
Anything bigger than 3000 pixels across is overkill since the system will compress it when you upload it. If you've seen an error and the file size seems reasonable, have a check that your image isn't 10,000 pixels across for example as that'll most likely cause an error.
Again, plenty of online tools to reduce the resolution if you don't have the software available to do this.
In short, any image you upload should aim to be as close to 1mb as possible
with a resolution no more than 3000 pixels across.
If you've tried the above and it's still not working, do get in touch with support and we'll be more than happy to assist in anyway we can.