I've been slowly collecting images to use for textures for new plants for a looong time, just need to finish the current projects, then I can help with this as well. At least I can offer what I have.
Some good places for good textures is a company called "Image Cell"
They have a whole range of commercial plants, trees, etc, that would be perfect for OFP's way of using flora. Too bad they're expensive as hell. They do have some free samples. Also if you look at their whole collection, you can grab all the images to use as reference...OR if your good, you can clone out the watermark and use them. Since this is not profit work, I don't see any moral dilema in doing this.
Another good reference is Turbosquid.com.
If you do a search for plants, many many images and textures will pop up. Many really nice 3d plants and their corresponding screenshots will pop up. I've bought some reasonably textures and models from here. If your serious about making new vegatation then this is another good place to look. If you search for free models and textures for plants, they do have a good collection of items.
Lastly, go to
www.gettyone.comI use this site for work on a daily basis, so I also do a lot of my own searching here "on the clock".
You can find lots of good images here as well. Anything you can think of. Again, this is another commercial site, and the usage right are quite harsh, so if you use a lo res version for something non commercial, be careful.
I have a lot of trial and error knowledege about making new plants...lot's of screwing around, so if you have any questions let me know.