Honestly, I've been encountering the same problem, Celebi in particular seemed to be a tricky one
My plan going forward is to get my images to be "Close Enough"
I've already had to use a generic Yellow Tiger for Raikou because I couldn't get decent images for her, I'll probably try again in the future
from what i learned. Yiffymix seems to take from E621's tags in terms of it's prompts. i think the way it works is that the more a specific prompt is present as a tag on e621. the more likely it is to get the right result. and plant OC's seem to be REALLY common... so that might be why Celebi seems to be tricky.
