Let’s say you have a rounded list and want add a larger image than the tradition ios 30×30. Try this:
1) Disable Smart Guides on the toolbar
2) add rounded list
3) drag-drop an image on to the left-side of a list-item
4) resize and position the image
5) if the image covers up the default label, select it in the outline and then drag to its new location in the list-item
See this Video example (no audio): http://screencast.com/t/ngVtHPLKU