Chances are, if the image that you’re looking at on you computer monitor looks fine, it won’t look very good when it’s printed.
Images that are design for the screen are very different from images that are design for print. For one thing, screen images typically have a resolution of 72dpi (dots per inch) while print images typically require more than 4 times that (at least 300dpi). That means that for every inch that your image spans the printed page, an image designed for print will contain more than 4 times the data , which translates to more than 4 times the detail.
Here are some examples for you: (These are all designed to show the relative difference between these three resolutions when printed).
600dpi (nearly all printers can handle this, some printers go up to 1200dpi):
600dpi (High Resolution)
300dpi (This is what I would consider to be the lowest resolution that one might want to use when printing grayscale or color):
300dpi (Print Resolution)
72dpi (This is an example of how images that you might pull off of the web, or from a screenshot will print)
72dpi (Low/Screen Resolution)
As you can see the detail progressively lessens as does the print quality.
The moral of this story is: When you pay a designer to do work for you, pay extra for the full resolution artwork. That way, when you have go to someone else, years later, to produce a print ad for you, you don’t have to tell them, “pull the images off of my website,” and they won’t have to try to explain to you why that won’t work.