Suggestion to generate a warning for the user if they upload an image too small

At the moment I can provide a guide via a Perch admin label but I cannot enforce anything or give feedback to the user if they do not follow these guidelines.

This would be especially useful since if users upload images that are too small for larger srcsets then crop for that sized image does not work (this confused me). This might not be seen unless you look at a Hi DPI device (I only caught this problem during testing).

With larger and larger images needed for Hi DPI devices and srcset I think this is going to become a bigger issue in future.