Hi Gigi,

You can replace those icons an item at a time with the icon field, or you can upload your own default icon so they all change. This is from the wiki:

Upload a 16x16 pixel image to appear as an icon beside the item name in the catalog. If no image is uploaded, the default icon appears beside this item.

To use your own default icon, upload the image. The uploaded image overwrites the default image stored in images/service_catalog/generic_small.gif.

Defining Catalog Items - ServiceNow Wiki