If I were to create something like this I'd want to make it as flexible and accommodating as possible so it could suit most people's needs and could easily be added as an upgrade even to existing sites that already have lots of members and photos.
And yes, the main image/window would be the full size pic version.
I really don't think this would be the way to go, the full size versions don't have any strict perimeter settings as for being vertical or horizontal, and also the default admin setting for the full size pics is a max of 800px wide and the vertical flow will be proportional to the original, which means a pic could land up being 800px by 1200px, which is just way too large to fit on a page as a photo gallery display application, and it also would be difficult to make the main display window expand and contract to accommodate the varying full sizes and still retain the thumbs and other elements on the page in a consistent configuration and appearance.
Therefore, using the medium thumbs which are a default size of 500px would make more sense. No matter what size the original photo is, the medium thumb version will always be a consistent 500px by 500px, and original images that are smaller than 500px by 500px, or not proportionate, would just be filled by the thumb's background color. jQuery photo gallery applications generally function and look better when the main photo pane is using images that are of a consistent size, and 500px by 500px is still a very generous view size, and of course they would still be able to view the full size version if they wanted to by clicking the image or link.
number of pics - never as many as 24.
I'd suggest (depending on pixels available up/down along the main large image, say maybe 5 or 6 per column, and 2 columns max (thus 10 to 12 pics max).
If possible I'd prefer to try and make it accommodate an unlimited number of thumbs and leave the restrictions as to how many are displayed based on how many photos are allowed in the admin settings.
Possibly 5/6 for free users.....and the extra column (i.e. a second column of 5/6 pics more) for paid users.
This would be a tad difficult, first off the natural flow and order of the thumbs is in a single row, when the number of thumbs reaches the threshold of the max width allotted for the thumbs to be displayed on the page, they automatically form a new row, also by default new photos added to the gallery are added to the beginning of the list, not the end. In other words the most recent photo added always shows first.
As for photo ratings and comments. I don't particularly need them......notably, I do not much care for the comments on pics.
I personally don't care about photo ratings or comments either, however a lot do, so I'd want to try and retain these options as well. They could easily be removed for those who don't want either one by simply removing the code for them in the HTML file.
I'd also want to try and retain photo descriptions, this is one feature I do like and IMO is more beneficial than even ratings or comments.
NOTE - for those reading this post, this is something that's just being discussed and kicked around at this point, and is only in the preliminary planning stages, so please don't start posting asking when it will be ready. It will be done only if I find it feasible and do-able, and if and when I have the time.