For the feature icons (citys, towers, mines, shipwrecks, etc.) and terrain graphics (the icons used for the mountains, hills, forests, etc. but not the background color), I'm using transparent pngs that are 250x250 pixels. I stay away from the far corners. These are scaled to 82% of the size of the smaller dimension of each hex then centered. So a hex that is 40x36 will have its icons scaled down to 29 or 30 pixels high (depending on rounding) and wide. Because the very top and bottom of the 40 pixel wide hex will only be 20 pixels, you'll want to avoid the extreme corners of your png. I also plan to allow users to customize the background colors of terrain and the scale percentage of terrain icons and feature icons.

Does that make sense?

Also, can you clarify what you mean by "not photorealistic"? Do you simply prefer a different style for all the icons and terrain graphics? If there is some constructive criticism, I'd like to understand it so I can make the software better.