One option is to overlay your graphic with a button. Set the button's normal style to no fill and the hover style to some other colour with the opacity set to 50% or less.
My graphic object is part of a map and so does not conform to a standard button shape? Is there anyway I can over come this?
Use the appearance settings in the inspector to insert the graphic object into a button. For the "hover" property, insert a different copy of the same graphic edited in a graphic editor to give it a "highlighted" look.