You have to supply the path to the photo file, not just the name of the file.
The easiest way is to place the image in a location which can be served by your Gateway - Inductive Automation\Ignition\webserver\webapps\main, for example - and then supply the path within that directory.
Assuming a file located at C:\Program Files\Inductive Automation\Ignition\webserver\webapps\main\carpic.jpg, you should be able to supply carpic.jpg for the image source.