Hi,
I think that you should be able to validate (in this or that extent) the response with the image using Validation functionality of LoadComplete (https://support.smartbear.com/viewarticle/65164/).
However, I think that you should consider if you are not mixing load and functional testing here.
It is functional testing (performed manually, or using automation tool like TestComplete) that verifies if the correct image is returned by the server.
With the load testing, you are verifying that the server can process the given load and the image is returned to the client. I might imagine that you may want to check if the same image is returned by the given response to evary VirtualUser, but I think that the more resource (or CPU)-friendly way is to check something like the value of the Content-Length property from the response header.