I would like to show certain content in the tooltip for all files of the "Images" group, including, for example, the file size, and possibly the x and y pixel counts/dimensions.
I'd be thankful for a quick note or a link on how this can be done for all members of the "Images" group in one go.
See if the .PNG file type has its own infotip defined. If so it will override the more generic group one.
It's also possible your PNG simply doesn't have any of the other data in it. Most PNGs don't as it's data that comes from cameras and cameras don't usually output PNG files.
Does any of the other metadata actually exist in the files you're looking at?
Thumbnails mode should still display the name and description, at least. You won't see the thumbnail in the infotip, since it's already shown in the main window.
It's not about metadata, but mainly about the image dimensions in pixels, and possibly file size. Also, in list view, the pixel dimensions (and other things) are already displayed:
The dimensions are shown in your screenshot, on the 2nd line, as part of {desc} (Description).
If you want individual width and height, or file size, then you need to add them to your infotip definition. Neither of those things are included in the definition shown in your screenshot above.
If you don't get any infotip at all in thumbnails mode, it's most likely because you've configured Opus to only display them in details/power mode here:
The lines are fairly self-explanatory, I think. As I said above, it's data that digital cameras add, so it's unlikely to appear for PNG files that probably didn't come out of a digital camera.
If all the data on a line is absent, that line will be skipped. But the data may still be useful for files that have it, so deleting it might not be what you want (unless you know you'll never need it, in which case delete away).