This is about photos in the Bird ID forum, not the Opus problem.
I think that the new photo attachment system is rather less efficient and more fiddly than the old one in several ways.
(This refers to my experience on a desktop Mac using Firefox on a 27-inch screen, 2560x1440.)
When I opened a thread that sounded interesting in the old forum, then I could look at the thumbnails and sometimes decide to look closer, sometimes not. And if I decided to look, I would click on the thumbnail holding down cmd and with my settings the full-size photo would open in a new tab. And I could open the photos that I wanted, click on the tabs to see them, and re-order or delete the tabs to have just the ones that I wanted. And with one click switch from the thread posts to any particular photo.
First annoyance: in the new system, the thumbnails are a square cutout from the photo, and sometimes the bird is only partly (and potentially, not at all) visible. So in many cases it will be necessary to open the full photos to judge whether to continue.
Second annoyance: the new thumbnail system will not open photos directly in a new tab by clicking one one with the cmd button down. Instead (cmd or not) it opens above and obscuring the text of the thread. It's true that there is a button in the top right to get all the thumbnails into this view, but the text is still obscured (and they are still the cropped thumbnails). Reading the thread text and then looking at one particular photo is still going to require several clicks instead of one.
Now, once I have got to this overlay view, there is now a button in the top right of this full-size overlay photo which allows me to open the photo in a new tab (window). But the overlay still remains above the text so I have to click on the background to get rid of it and read the message again. So, it's several clicks, rather than one.
So: with some effort, I can still do what I used to do - have the thread in the first tab and each photo open in its own tab.
So, perhaps not such a big deal.
Annoyance three: I, and quite a lot of other people in threads I have taken part in, sometimes drag unclear photos to the desktop and open them in photoshop to lighten them or darken them. Occasionally these 'improved' versions are posted back to the thread; in my case at least, once I have used them for ID, I delete them from my system. This method doesn't work with the new full-size photos And the download button which shows in the overlay view doesn't work - so why is it there?
In post #4 above, Deb suggests that this protects posters' copyright. But it doesn't. Because all you have to do is to do a screen grab over the photo (on a Mac, cmd+shift+4, then drag over the area you want to copy). I don't know what the new maximum uploadable image size is, but when I used this method it got me one photo at 1700 pixels wide.
(And to further counter Deb's point, the old drag and drop to copy to the desktop works fine for all the photos in the Opus and Gallery, which is where most of the photos likely to be 'stolen' are; and you don't need to be signed in to see the full-sized photos there. And if this were changed so you did have to sign in, then what would happen to all the Opus articles and Gallery photos that come up as a result of regular online searches from over the world - and draws new members who make their first visit to the forum because a search threw up such a link?)
So far: it's maybe just me being annoyed at trivial (but time-consuming as there are quite a lot of extra clicks involved) changes to my working habits.
But where I think this will be a problem for more people is when you get a long thread, and especially a thread where several posts have photos. Because here, I might have several photos from different posts in the thread all open in their own tab, and want to refer simultaneously to the thread itself, so I can remember which post each came with (which will be even more difficult with the new thumbnails not showing the whole photo because in some threads it will make it difficult to match the photo to the thumbnail). Again, I'm sure there's some kind of workaround, but it's going to involve lots more clicks and faff. For me, at least, the old system worked fine, and the new one will be less satisfying.
Bonus point: in the post/reply box, there is a button at the bottom to 'Attach Files' in the old way which produces the new, less helpful thumbnails. But there is also a button with the editing tools at the top of the page to put a photo directly into the post, not as an attachment (and another button to directly insert sound or video). This has already been used since the new site was done (see the thread 'Three from Cora Rica'). This doesn't affect me on fibre-to-the-home at 10GB/sec, but surely it's going to annoy people in less-well-served places and phone users who will end up downloading full sized pictures for every thread which puts them in directly and uses up their data allowance? Interestingly, if the photos are directly in the post, then cmd+click will open the photo in a new tab, but this is less necessary because the photos are full size already in the post itself.