Sheet Being Rejected for Being a JPEG, But Is Exported As PNG?

[Resistiv]

Yep, I was investigating further after I posted this thread and realized what had happened. It's most likely due to the screenshot technique I used to rip the sprite. However, I find the artifacting and color variation very strange, as Windows natively saves all screenshots as .PNG's if I'm not mistaken.
Also, the game is Dragon Ball Z: Attack of the Saiyans for the DS. 
Do you have an advice regarding hard-ripping that would help minimize such occurrences in the future?