Image resolution describes the detail an image holds. The term applies equally to digital images, film images, and other types of images. Higher resolution means more image detail.
Image resolution, in print, is measured in DPI (dots per inch). Below, images are presented in different resolutions from lowest to highest.
High-resolution images are the best for truck-side advertising because when they get "blown up" to fit on the side of the truck they do not get as "pixellated" or blocky looking.
Image and text adapted from a Wikipedia.org article accessed on August 28, 2007.