I have recently encountered this issue where in a plugin image was not transferred along with the plugin step in the deployment.
The plugin step was profiled in the source environment and the solution containing it was exported and deployed to other org.
I have a feeling if your plugin is profiled, the image will not be taken to other environment. Obviously it was a miss from the developer but this is something new i saw.
All i did was unregistered the assembly from target environment —> unprofiled the step in source system — >exported the solution again from source — ->deployed the solution to target.
Hope this helps!