If your image appears, but then is gone when you come back to your form later or when you refresh the page, then usually it's because the image upload has not finished before the page has been refreshed or closed.
This happens more often when the image is large, as it takes a while to upload. Make sure the image that is uploading is not slightly grayed-out (this means the image is still uploading) before you refresh or close the page.
Another possibility is that your image is too large. Images over 6MB usually fail, so keep your images under 6MB.
If you are still experiencing issues, please let us know at firstname.lastname@example.org.