Wow Powerpoint !!! I gotta rep your perseverance...

I would try Gimp. Its a bit daunting to start with but you dont need to use every last button and widget.

If your using art that needs to blend then you need to use PNG type images which have transparency but some of them just switch it on and off and other images fade the blend up - you need to get some of those, or make some up.

A lot of art programs like Gimp and photoshop work on layers so what you can do is put water in on one layer and grass on another upper layer so that grass overwrites the water. Then you can take a digital eraser and pull away some fo the grass to leave the water underneath. When your happy then save that out as an icon. Or leave the background clear, erase some grass and pop them over water and it will fade up grass. Just remember to save these images as PNG with all the 'alpha' or transparency information.