A pair of lady birds here that can signify love between a couple or two good friends enjoying the company of each other. So your message can be 'I enjoy the time we spent together.' But you must remember not to make your message long because a river stone of this size may not be able to hold your entire message. And of course a short message will carry a stronger impact.
You can actually paint your river stone to look like lady bird like an example here.
Eh... this is not a stone but an eraser dust cleaner. This is just an example of how you can create an animal or an insect using the entire river stone. I do in fact have a river stone similar to the photo above but I don't have a photo shot of it as I have left it in my husband's office. I certainly hope that the photo above would give you an idea.
The other method is painting a scenery on it. I had painted a night thunder storm on a flat river stone. It was stunning. Again I am sorry that I don't have a photo here. The stone is in my husband's office too. Yeah, he is my greatest supporter of my art.