As far as I know, it is not possible to validate images in LoadComplete in the way you are describing. Like Alex mentioned, this sort of validation is better suited for functional testing with a tool like TestComplete.
However, if you just need to verify that the server returned an image file, LoadComplete will do this by default. Looking at your screenshot, your scenario includes a request for that image and expects an HTTP status code of 200 in response from the server. That means that if your server fails to return that image, LoadComplete will flag any response code other than 200 as a warning. (Additionally, you can specify how LoadComplete treats different response codes for this request.)
If you'd like, you can also do some manual spot-checking to make sure the image is being returned as expected. To do this, enable logging of request bodies by going to Tools > Options > Simulating > General and checking the box next to “Store full headers and bodies.” After doing so, you will be able to consult the Details view of any load testing report to see every image that is returned from the server by looking at the response bodies for any given virtual user.