[Opserty]

You could use the "Regular Expersion" check on it (located on the page where you add/edit the field). Be best to check on google for the code to input in that option,, something like "regex valid URL" would probably get you some results.

Something I found from a quick search:

^(http\:\/\/[a-zA-Z0-9\-\.]+\.[a-zA-Z]{2,3}(?:\/\S*)?(?:[a-zA-Z0-9_])+\.(?:jpg|jpeg|gif|png))$

Not to familiar with regular expressions myself but you could try that out and see what happens.