The style of your symbols is not photorealistic. Roughly how many pixels across are your hex symbols, so we can get an idea of how much detail to include?The custom icon graphics will probably expect simple pngs. You’ll be able to use any size you want as the software will scale them to fit within the hexes.
My to do list includes making some webding style symbols for modern/scifi units for my own use. I might make them Creative Commons and, if so, Hexographer users might be interested - though with my current schedule someone else will almost certainly beat me to it.
I've seen some on the net, but I've found no decent free ones yet. If anyone knows of some it will save me some time and may help Hexographer users.