Im getting the 'Cheatin' uh?' error

Can someone please assit me with this problem im having with my site.. i’m trying to add pictures to my galleries and when i try and add a preview picture its giving me an error saying ‘Cheatin’ uh?’ and i don’t know how to get it off my screen so i can continue to update my site retire248.com. I’m having the problem in my video section (https://www.retire248.com/videos) and my picture section (https://www.retire248.com/gallery). Please someone with knowledge of this error please help me out!!! Thanks a billion…