The material gallery effects work only on items that are paths, not raster images. Is this a shape you created? If so, you can turn it into a path by using the Edit/Trace Edges command.
I have a couple tutorials here on SBB that cover this:
Tutorial to Rasterize an Image
In this example it shows how to change combined shapes that have been rasterized into a path, but the same technique could be used for a .png. Sometimes you'll need to do a bit of tweaking on the path to smooth out the edges, but generally it works well.
You can save it in the shape gallery as well if you want to use this shape again and again with the presets. Here's an example on how to do that, only I started with one of the shapes that PI came with and did some modifications. Once you turn a shape into a path though, you can do the same thing as the egg example I used.
Saving Custom Shapes
Hope this helps! PI is so much fun, I love it!