In order to reproduce a Photo Realistic Image we require a 300 pixel per inch file at actual size (this includes any photoshop files our scanned images).

To understand why this important, try thinking of resolution in terms of volume.

Lets say we have a clear 10oz bottle filled with blue liquid.
This represents the amount of information in a 72 pixel per inch file downloaded from the Internet at say 2" wide.

Now what if we were to increase the size of the bottle 180% but we did not increase the amount of blue liquid inside the bottle.
This represents what we are trying to do when we enlarge that 72 pixel per inch file from 2" wide to 10" wide.

Our bottle would be more than half empty.
What we've done is enlarged the parameters of our original image without adding any more imaging information.

The computer attempts to extrapolate (compensate for) information that simply isn't there.
This is what happens to our bottle of liquid.

There simply isn't enough information (volume) encoded into our original file to enlarge & print a clean, perhaps even recognizable image at full size on our product. If we have a true vector art file (i.e. adobe illustrator file with text converted to outlines & no placed images) resolution is not a concern.