whats mean "Cover set failed"

i have upload file and i read this message :

Your thread need to be moderated, it will be displayed when verified by moderator
Cover set failed.
whats means  "Cover set failed"?  i mistake something? what i must do?

thank you

greenteas

your thread need to be moderating

your cover is not included
maybe it was too big
just go to your thread-EDIT-and upload a smaller size of the cover