There are two areas on the ask a question where this type of reminder could (and I'd argue, should) be included.
When you first open that page, there is an info box on the side of the page that reads:
How To Ask
Is your question about graphic design?
We prefer questions that can be answered, not just discussed.
Provide details. Share your research.
If your question is about this website, ask it on meta instead.
In comparison, the info box when asking a meta question instructs users to "Provide details. Show examples" instead of "Share your research" -- a wording that is probably more relevant for graphic design. But something even more specific, like "This is a graphic design site, so include example graphics" would be better.
It won't catch every one -- the info box is somewhat tucked away on the side of the page -- but it might wake up a few people.
On the same page, when the keyboard focus moves to the actual compose-your-question text area, the "How To Ask" info box changes to a "How to Format" box with a list of common formatting tasks. But it doesn't mention how to insert images, so new users might not immediately realize that this is a possibility.
The list of formatting suggestions can be customized by site (e.g., other sites include information on formatting code, math equations, etc.). For graphic design, a mention about how to upload or link-in images seems sensible. Again, it won't eliminate every "Can you please upload an example?" comment, but it should help.