Check if an image has loaded

I have a users table with a varbinary(max) field containing a photo of the user.
My profile page takes the username from the query string and builds an image url querystring and the onload event sets thge image controls source url to that.
All fine if the user has added a picture, but I have a place holding image I would like it to show if there is is only the ubiquitous red X in the image control.  How do I test if an image has been returned?

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