In the text inspector click on the list tab. Here you can choose different types of bullets, including custom images. The default is text bullets & in the small drop-down menu above the color box, you can select from several different ones.
I frequently use bulleted lists in my church newsletter & have several set as paragraph styles to easily reuse. You probably would want to set a paragraph style if you used a custom image & would like to use it again.
Peggy