With more people using iOS devices including iPhone, iPods, and iPad and whatever else might come along it is a good idea to make sure that you present your site in the best possible light for those devices. You might be aware of the favicon.png which is documented here. However; when an iOS device comes to your web site there are additional images that can help differentiate your site.
For iPhone 4 and current generation iPod touch you can use 114px x 114px, while iPads use 72px x 72px and other iOS devices use 57px x 57px. You could create a unique image with each of these sizes or just create one and let the device scale as needed.
The way to add these special icons is to add the following to the top of your web site. Learn More