Would it be better if:
A. fewer pictures of each plant
or
B. same pictures, only smaller in size that link to larger versions?
I think the pictures can be valuable so I wouldn't want to lose any of them.
The large pixel size usually helps, too, especially on a phone because their pixels are smaller. A PC screen is usually 96dpi; An iPhone is about 160dpi and my N97 is over 200dpi. So a 500 pixel image is about 5" wide on a PC but less than 2.5" wide on my phone. You need good eyesight with small images, so I wouldn't want to make yours much smaller.
I'd be tempted to experiment with higher JPEG compression, for lower quality and quicker downloads at the same pixel size. I don't know how to go about doing that with Flickr, or indeed other online hosts. Obviously it's more work.
I notice that you've reorganised it so that the pictures aren't all on the same page. That helps download times. Previously when you clicked "Woods", you got pictures of all the woodland plants, and that was a lot; that was what really struck me in my original comment. Now you just get a list of names, and you only get the pictures for a plant if you click on a name. That's quicker for downloading, although less useful if you don't know the name and are trying to identify something visually.
I doubt there's an easy answer for this.