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I tried uploading pictures using the form cover and it showed up fine. It was uploaded successfully.
I would recommend you to log out from JotForm and then clear your browser cache. After that, you need to close and re-open the browser then you need to sign back in. Please try to upload a picture again in the form cover. If it is still the same, try doing it on another browser.
What type (jpg, gif, png) of image are you trying to upload?
Hope this works. Let us know if it is still not working.