Yes—typically, a Canon 7D’s 18MP file is more than enough for billboard use. Billboards are designed to be viewed from a long distance, so they are printed at much lower resolution than close-up prints. In practice, billboard output can be surprisingly low, sometimes around 9 dpi, because viewing distance does most of the work.

The exact requirement depends on the billboard’s physical size, the printer’s specs, and how much fine detail the image contains. A simple, bold image is easier to enlarge than one with very fine textures or small text. But in general, resolution is much less demanding for billboards than many people expect, and an 18MP file should be sufficient for most billboard applications.