We all know that a high quality image can add a lot to a design in terms of style and depth. However, when designing a banner there are somethings to consider before using just any old image out of your point and shoot or getting a photo off google images.
A banner is a much larger canvas then a normal photo would be printed on and most images on the web are optimized to load quickly, which means small image-size and low resolution.
If you were to do a google search for images of puppies, chance are most images would not be a high enough resolution to work on a banner. For example the image I have selected is 500x309 pixels, this makes it about 1.75 x 1 inches, which would not work. If that images was at 300dpi, and we scaled it to our minimum 75 dpi it would be 6.5 x 4, which still would not work. Keep in mind that since most images are optimized for the web they will be at 72 dpi.
Digital cameras have come a long way since they were first introduced. If you are using your camera on the highest allowable settings then the image might be a high enough resolution to be blown up on a banner. However, chances are your settings are set lower then the maximum so that you can store more photos on your memory card.
A good alternative is to look at is buying stock images to use for your banner. Two good placed to start are www.istock.com and www.shutterstock.com. Both of these places allow your to purchase images at a reasonable price, they are easy to use and have tons of photos. If you are having a hard time finding a photo you could also take is to hire a professional photographer to take photos for you. This way you can get the exact pictures you want.
Just some things to consider when you are looking for an image for your next banner.