Everyone's tried to print an image off the web, only to notice it looks horrible on paper vs. the screen. Guess what? It's not your printer, it's most likely the file, because your file is the wrong dpi/ppi (dots per inch or pixels per inch). Here is a quick print primer lesson from The Photo Ninja.
Alright try to follow me, i'll keep things painfully simple. Your computer monitor shows images at a web resolution of 72 dpi (that's 72 dots per inch). A common photograph print resolution is 300 dpi (that's 300 dots per inch).
What does this mean?
Basically, you can't print images off the web and expect them to look what you see on the screen. Most likely, what you will get is a pixelated image (see attached sample @ 72 dpi).
How do you solve this problem?
Always use the original files captured by the camera. They should have large file sizes, since they haven't been compressed for the web. Also don't steal photograph files you don't own the copyright to and you won't encounter this problem as often.
How to find out what dpi a file is to ensure it prints correctly?
On a pc: Right click on the image, and select Properties, then on the top tabs choose Summary, it should display it here, if it doesn't then click on Advanced which should be on that summary tab.
On a mac: Hold Apple + i (or command + i). This should pull up an info box.
Etc: Image editors such as Photoshop Element or Paint can tell you the dpi of a file. Click on Image info.