Can’t Upload a Photo

I have created a custom photo upload field and placed it on the registration page. In testing it, whenever I try to upload a photo, I am told “Your photo is too small. It must be at least 600px wide.”

I cannot find any field that controls the minimum size for an uploaded photo. I have reset to their defaults all of the values in Settings | Uploads, even though none of them speaks to this.

This is doubly curious because it is an absurd minimum value — for showing this as a thumbnail or on a phone, there should be no minimum, and if there were, it should be 100px or so.

Can someone help me figure this out? Thanks…