Anna, If Sue or another database administrator isn't around, you can delete the photo yourself.
Go here:
Tweedy's ticktrefoil (Desmodium tweedyi)
Scroll down and click on your photo; then scroll all the way to the very bottom of the page and click on where it says "Propose this image be deleted". It will ask why and just type in that it's the incorrect plant and then click on submit. You will get a system message when it's been approved and deleted.