Something like UploadWizard's, but ignoring the complications in it that aren't true for cross-wiki ("with permission"/PD), be clearer about license and irrevocability, and less cartoony so that people actually pay attention:
Thank you for donating an image to be used on Wikipedia.
You should only continue if it meets several tests:
[icon] It must be entirely your own creation, not just taken from the Web
It has to contain no work by anyone else, or inspired by them [icon]
[icon] It should be educational and useful for teaching others
It must be OK to publish forever on the Internet under the Creative Commons license [icon]
Possibly these could be checkboxes people have to confirm for each, with bail options taking them elsewhere (e.g. other people's work -> UploadWizard). This is intentionally harsher than the laissez-faire attitude Commons has about taking a lot more, because it's better to verge on the side of caution.