A super-handy source for botanical information, (and open-source plant images) is the USDA’s plant page: there’s tons of pictures so that it’s easy to figure out what a plant really looks like, info about its distribution, endangeredness, toxicity…pretty much whatever you need to know. It’s way more comprehensive than anything I could put together. Here, you know what? I’ll just give you a list of names and you can go learn about them over there.
If you’re wondering whether a given plant is toxic, Cornell University runs a very well-designed page on poisonous plants that is searchable by latin name or common name. It also indicates whether plants are toxic to animals other than humans, so for example there is an entry for “Allium” that indicates toxicity to farm animals, even though alliums (onions, garlic etc) aren’t toxic to humans. The Poison Garden is also an excellent blog covering the botany, history and use of all types of poisonous plants.
A lot of the plants I post about are robust California natives, which means that in the context of gardens and agriculture, they’re weeds or pests. UC Davis runs a great website about integrated pest management, which covers animal, botanical, native and invasive species.
For botanical illustrations, the Smithsonian’s catalog has a very comprehensive collection, but its utility is constrained a bit by the search tools: you have to look by genus or family, and they’re in drop-down menus. I think being able to type the name of the plant into a search field and having the entries cross-referenced by common name would be way easier (hint, hint, Smithsonian).
An artist I’ve started to especially admire is Herman Adolph Koehler, whose 1887 work Medizinal-pflanzen naturgetreuen (Medicinal plants in nature) is really beautiful, and thorough, with cross-sectional images and details of all the plant parts. If anybody feels like dropping $1000 or so on buying me a present, get me his book and I will happily frame every page and cover my house in them. Some of the pictures on my header are excerpted from him. There’s a lovely catalog of the digitizations of his pics on Kurt Stuber’s webpage (and I apologize for the lack of proper umlauts. I have to figure out how to do those).