How to Clear Photo Copy Box

I tried to post photos of my sick boxwood, but uploaded the wrong file. Apparently, I cannot upload the correct photo because the Copy box (dialog box) continues to hold the wrong photo info. I cannot get it to clear. What to do...going crazy! Thanks.

Just leave the message you were posting and start over with a new message. If you had already posted a picture you didn't want to post, click on "edit" at the top right of the message screen, then delete the address of the picture you posted, then try again.