The Memento DVD cover is made to look like one of Leonard's Polaroid pictures. It's got that Polaroid quality to it and even has the title scrawled at the bottom to look like Leonard's handwriting.
Is the cover suggesting that the film, to us, is like the pictures are to Leonard? Well, sure. The film gives us brief snapshots of a timeless (in the literal sense, although the movie as a whole does seem timeless) world that we can only assess through what we've been shown.
But what's most interesting about this cover art is something called mise en abyme (which is a French term meaning, literally: placed in abyss). It's the effect of repeating an image within itself, creating a pattern that would become infinitely small. The same effect occurs if you're standing between two mirrors.
The importance of this effect within the movie itself is more abstract. It represents the separation of Leonard's conscious self from his past selves, and it represents his inability to have a continuous identity. It also points to the futility of trying to record time itself in pictures… because even photography can't escape the subjectivity of the photographer.