University of Wisconsin-Madison’s Dept of Botany have many teaching images available on their Virtual Foliage homepage. They are divided up in three themed sets (linked below with an example from each).  It is a little cumbersome to search through various folders, but they cover a broad swath of topics and taxa.  This resource was developed largely by Mike Clayton, now retired, and some of the names are outdated, but the images are of course very useful. (Thanks to Ken Cameron for sharing this resource.)

pentarch anatomical structure in stem

(above) Pentarch anatomical structure in stem; General Botany images, (c) Mike Clayton, UW-Madison

General Botany:
Image folders: Angiosperms, Bacteria, Botany on Campus, Bryophytes, Cells and Tissues, Chlorophyta, Diffusion, Diversity Exploratory, Eukarya 1, Fungi, Gymnosperms, Heterokonts, Lycophytes, Meiosis, Microscope, Mitosis, Modified Plant Parts, Organic Molecules in Plant Tissues, Photosynthesis, Plant Cell, Plant Morphology, Pteridophytes, Respiration, Root (Primary Growth), Shoot (Primary Growth), Secondary Growth in Stems and Roots, Symbiosis, Tree Identification on Campus (Wisconsin, USA), U. W.  Arboretum.

Fern gametophytes with sporophytes

Fern gametophytes with sporophytes; From Plant Diversity Images, (c) Mike Clayton, UW-Madison

Plant Diversity:

Image folders: Equisetophyta, Lycopodiophyta, Magnoliophyta, Pinophyta, Psilotophyta, Polypoliophyta.

Catalpa speciosa leaves

Catalpa speciosa: Trees of Wisconsin Images, (c) UW-Madison

Trees of Wisconsin:
Image folders: Many, arranged by genus.