Changing Icons on the "Up Ahead" feature

Has anyone changed the ICONS on the Up Ahead feature?
Say, if you want to choose a specific gas station like "shell" or a specific restaurant like "McDonalds"

In the up ahead feature:

* To search for a specific business or category, select a service, select Custom Search, and enter the name of the business or category.
Select Done.

I know that when you enter "Shell", the icon automatically appears instead of the next step, which is selecting from the generic list. But none of the others that I try do this.

Does the icon need to be a specific size for the up ahead feature?

Thanks!