Samsung MM-A900 / SPH-A900 / Ace

I understand how to get my music, pictures, and videos on and off of the phone. But my problem is when I try to take the pictures and videos off of my phone to save memory. I have been hooking my phone up to my computer and going into the media files and deleting them, but then when I look at the amount of memory left on the phone, it has not changed at all. And the pictures & videos are not in "My Album" and there not in the "file manager". How can i get my memory back?? Any help would be appreciated

Thanks,