Dear Steve,
what is the CRED version you are using?
I'm getting a 404 message when uploading the image in the form you mentioned. Do you know if the your server has mod_security or similar apache module that can be preventing the upload of images/files on frontend? If this is the case the resolution for this can be to ask your host to change the rules for this site in order to allow the uploads.
Please let me know if you are satisfied with my answer and if I can help you with any other questions you might have.
Regards,
Ana