If I have pictures of a specific plant on the data base and have newer pictures that are much better should I delete some of the lesser pictures and add the better ones or just leave the first ones there and add the better ones?

The latter. Keep the old photos in there and add the new ones. The new ones will get thumbs from the other members and that'll cause them to appear higher on the page.

What is the rationale for keeping an inferior photo of much the same shot? I'd delete the first one myself. I guess it is up to the contributor, though, correct?

I guess the rational is that every photo has some value, assuming it's not a terrible shot. Every picture gives more information about the plant, whether it's a different angle, capturing a different color or season, and so forth. Putting yourself in the shoes of a visitor who was googling for information about a plant, wouldn't you like to see 12 photos instead of 11?

Not really. I find too many almost identical photos to be distracting and just kind of clutters things up. Perhaps it's just me. I pass on adding my own photos when I see that there are already several similar to mine. Which then causes me to think a bit outside the box and perhaps go out and take a photo of a single leaf, the backside of the leaf, just the bark, etc. I would rather see the photobase grow through diversity rather than sheer volume. (No offense to anyone intended, they are all lovely photos.)

Diversity definitely trumps sheer volume.

And we also have a "Page 2" feature where near-duplicates and blurry photos, etc, can be put. Then they are still visible but not on the front page.