I have been using this site religiously for six months and I love it! One thing I really want to see added on would be plant life. I was thinking perhaps close-ups of different types of flowers and trees and such, but whole backgrounds would be nice as well. Perhaps that could be two new categories - 1. Plant Life and 2. Backgrounds
I have been experiencing the same problem. This site is great for animals and I've improved immensely with them since using it, but doesn't do much in the way of other things. I tend to go somewhere else now for figure drawing models as well.
I use free-use image site like Pexels, collect 15 or 20 images of the subject I want and then time myself.
I am finding a lot of 'generic plant slideshows' on YouTube. You could always do that and pause it if you need a bit more time for a given image. With regard to backgrounds you could always find a image-centric blogging platform like tumblr or pintrest and collect stuff there for sessions.
I started on an "environments" drawing tool ages ago, and will likely return to building that in the near future. I don't know that we'll do a whole "plant life" specific tool, but a very popular request over the years has been still life and maybe plants could be a category within such a tool?