I didn't see any rules setting the Thumbnail field - I do see it has a default value. I also in the rule inspector didn't see anything setting the Hyperlink field. Am I missing something here?
I guess what I'd do is put the Thumbnail field on the form again, as a text box, so you can verify the URL it is getting set to. Like I said - this behavior didn't reproduce for me in a test form (using a default, non-calculated value for my picture URL). If you just open the form as it is, with the default URL you currently have in the Thumbnail field, do you see the pic?