Yes, you can use your own PNG images of plants with the Land F/X Sketchup Connection. As long as you've already modeled them into face-me components, you can assign the component file to a plant using the 'Browse' option on the Sketchup side.
Here's a short Power Tip video on how to assign this component. All the same rules apply to a face-me component as with a custom 3D component. You still have to make sure that the axis in the component file are oriented correctly so that the tree doesn't come in upside-down or sideways.
Power Tip: Assign a Custom Sketchup Component to a Plant