Is there a way to convert a low resolution photo to a high resolution photo for iphone 4?

I've heard that people add the @2x. thing behind the photo's name to get it in HQ. Does it actually work? If it does, what do i do? Do I just simply transfer both of them or what? And if it doesn't, what can I do? I need clear and direct instructions. (I suck at this) Thanks in advance.