WebGen2 gets my vote, although it is a bit daunting at first because of the number of options. I have used it on my website - [link]
(click on the gallery)
Webgen2 does produce rather a lot of files if you use the simple options - approx 5 per photo. The Java script options produce fewer but I have avoided these
because they don't work with NetSurf.
It is fairly easy to produce a stand-alone set of pages containing a good looking set of thumbnails and corresponding pictures including simple navigation controls.
There is some provision for integrating these pages into an existing site, but I find this needs some tweaking.
Making changes to the generated html is possible but it is usually easier to regenerate the pages and re-integrate.
If I could request one change it would be for the default files to be htm not html