Hi there! First, I want you to backup your videos and pictures. After that, try going to Settings ➔ Applications ➔ Manage Applications. Go to All tab. Look for Gallery and tap on Clear data.

I have a zte majesty phone with straight talk & am having problems deleting pics..videos..etc. I trid going to file manager to delete them but they always show back up. My internal memory & sd card are practically full & it's slowing everything down. Already moved everything I could to sd card.