I have been adding "comparison photos" in threads attached to a few of my database images, for example this recent one:
from
The thread "Winter comparison" in
Photo of Aeoniums (Aeonium)
These two photos of the same plant are spaced 6 months apart.
This is a different kind of comparison than what William was talking about, but I have found it helpful to show seasonal variation, or growth over time, in a way that associates individual database photos with comparison shots of the same plant. If there was a formal way to link database images for comparison, that would presumably accomplish the desired function, either way. Seems like it might involve a fair amount of work, though.
Multi-plant comparisons tend to work best in the Plant Combination format when they are in pots and can easily be moved together. I don't see it working all that well for landscape plants that are not side by side, or flowers appearing at different times, for example. Here is one I made to show the differences between 3 types of "old man" cacti:
