If you had set the data in the model or controller you could access it. So you'd have to think ahead in the controller. As you said you can't load it in the controller, perhaps you need to write a specific controller function, which you can call from the view using $this->requestAction() and pass in the image name or similar as a parameter.
The only disadvantage of this is using requestAction() is frowned upon, as it initiates an entirely new dispatch cycle, which can slow down your app a bit.
The other option, which may work is creating a dynamic element and passing in a parameter into the element and have it create the image for you. Although I'm not too sure how this would work in practise.
How are you generating the thumbnails using the helper in the view if you aren't passing data into it from a controller or model? I mean if it was me, I would be setting the 'database resource' in the controller, and passing it to the view that way, then having the helper deal with it in the view. That way you could bypass this issue entirely :)