Well, it's (sadly!) a bit more complicated than it used to be (and limitations of the forum software prevent it being quite as clever as I would wish)...
At present, attachments (like images) of up to 1000 kB are permitted. However, there's an extra layer which I turned on recently, which sort-of hides this filesize limit for some images: if either the width or height of the image is more than 2048 pixels, the image will be auto-resized (when it reaches the server) to have a maximum dimension of 2048 pixels (preserving aspect ratio of course) and the image quality settings for this conversion are such that I've yet to find an image that is over the filesize limit after the scaling.
The unfortunate bit is that if
neither dimension exceeds the 2048 pixel limit, the auto-resize isn't triggered and then quite a few images will hit the filesize limit
It would be nicer if the server downsized big files to meet the filesize limit too but that doesn't (yet?) seem to be possible.
Having said all that, I run at 1080p myself (i.e. under the 2048 pixel limit), and generate all of my AC screenshots with Steam (via the F12 key) - this makes a JPG file in a slightly-irritating-to-find folder.
I've not yet seen one that exceeded 1000 kB - I just took a peek and the current set of 1400 of them that are sitting there are mostly around the 500 kB mark with the very biggest of all coming in at 862 kB... (I don't think Steam offers a quality setting but the default quality looks fine to me
)
Anyone creating 1440p or 4k images will of course benefit from the auto-resizing mentioned above.
Clearly some 1080p-ish screenshots
are exceeding the 1000 kB limit of course - I'm curious to know what app is creating them.