Return different images based on URL

Hello:

I have a table with a list of agents, the table has a column that has an image url for the agents.  Not all agents have an image therefore the field is blank.

I'm using a visual that can pull in an image based on a url field. 

I'd like the have the agent image show if there is a valid url in the field,  I'd like a different image to show if blank (I'll probably through a "No Image Found" image url out on the web somewhere).

I can't figure out how to do this or how to create a measure that would do this evaluation.

Any help would be appreciated.

Thanks!