For years, I've been using the Flickr Classic Plugin, and it's always worked beautifully, but unfortunately it's been discontinued. So I need a replacement.
Finding an extension that can pull images from Flickr, and show them either as a gallery or as a slideshow is not as easy as I thought.
SmartSlider can do it, in fact I am already using it on a site that uses SmartSlider anyway, but I don't think SmartSlider does galleries. And anyway, it's way overkill if all you want to do is show a simple gallery.
Then I tried SigPlus. Sigplus can pull the images from Flickr, no problem, but I don't like the way the layout looks at all. It can all be overridden with CSS, but that would signify an awful lot of work, and the result might still be a bit wonky.
Next up: Event Gallery. Overkill as well, because Event Gallery has a complete system in place for selling images - but it does in fact display a gallery very nicely. I also managed to create an autoplay slideshow. At first, the images jumped across the screen at an alarming rate, but I managed to tame it, see Trying a slideshow. Not as pretty as the Classic Plugin slideshow, but I probably need to spend more time tweaking it.
Event Gallery also works flawlessly in J!4.