Okay, I have this working now. If you know the plant ID of the plant you want to import an image over to, you can enter just the ID number and it'll find it for you.
How do you find the ID number? Go to the plant in question, and load it in your browser. You will see in the URL field a number after "/plants/view/" and that number is the ID of the plant.
The address for the main Rose entry is:
Roses (Rosa)
and therefore the ID for the rose entry is 181506.
So go to import an image and enter that number in, and it'll find the rose entry for you.
Let me know if you have any questions about this!