It is indeed limited to the main image, because additional images open in a new window as a "raw" image: there is no page around it to add any control.
I could add something to open them in a javascript popup, to have the ability to also display a brighten button. That would also be more handy as you can easily close it (rather than having to go back to the page).
If one wants to open an image in a new tab as currently, it would still work, but without brighten button though.
I've added it to my to-do list, but there are a lot of things on it

I noticed that the ability to brighten an image is limited to the main image, and not the thumbnails. Should this ability be extended to thumbnails as well?

It seems to work now, but with slowdowns at some moments.
I activated the bot filter, as there are many on the site as often

Such issues are rather for the "Site/Ad Issues" thread.
Here it is the thread for the changes I made and the rants about them

Site is unusable again 8.30am Europe time.

Fixed, though that the flag for the car is probably a little too low (but it is like before :D)

They were misaligned in the movie title bars, but I didn't notice that in the car model name they were OK before.
I'll revert to the old alignment for the car model name.

The layout of the flag icons and words seems a little bit off, I don't remember they look like this yesterday.

https://www.imcdb.org/vehicle_847719-Alfa-Romeo-Sprint-902A-1983.html
Just for reference, it may not be a good comparison since I can't found a recent archived car page, but the layout was normal even after the big update recently.
https://web.archive.org/web/20230707045101/https://www.imcdb.org/vehicle_847719-Alfa-Romeo-Sprint-902A-1983.html

What do you mean by "the larger font set"?
It is the same font as other fields and the text of the page, and (should) becomes larger when you change the text size in your profile (not the management page setting).

When releasing the new version last week I briefly increased its size, but afterwards I changed it back (when tweaking a little the field sizes, to address complains), thinking there was no reason why specifically that one would be bigger.
In the old version of the site, it was depending on the OS/browser, like other fields, so for some it was bigger and for others it was a similar size.

One thing that I have noticed, is that when you are writing a comment, the font size is the small size and not the larger font set. Or is that part of the "Management pages" setting? Because I had that set to 100%?

Indeed... but that's not very useful to collapse it, as it will be collapsed again next time you open the page.

If you wanted to see them you see them