Maybe I'm a bit to generel (or even maybe to restricutive) in my considerations not only referring to photo stories posted in a forum but also installed on Web pages ... anyway my thoughts about:
I personally distinguish between two types of photo stories (at least those I make): the "real" photo stories (according to my understanding) and the illustrative or documentative one.
The first type does not need any caption because everything is included within the photo itself e.g. in a comic book style with speach bubbles (e.g. cf.
here). Arrangement is quite simple because you can post photos one after another (either on one page all or in a squence of pages, each single on one page).
This is the one I prefer because everything is compact and it's quite clear who is speaking an acting. I also like each photo to be presented on one page because you are not mislead for a preview by a simple scroll. Possible drawback: if during setup you do not have in mind where the bubbles later will be, there might be some details of the photo hidden; but if you are thinking before on the storyline and the setup this can be avoid quite easily.
For the second type -- illustrative/documentative photo story -- I use captions below like (as already said) in books. Reason for me is, that at this kind of photo story the picture itself stands in the foreground (e.g. cf.
here), i.e. it could stand for itself and the viewer only get's some additional information by the caption, so it is not important if he reads it or not (e.g. because he/she imagines by viewing the pics his/her own interpretation or "story").
(For me it's quite "exhausting" to read photo stories where photos' content only differ slightly and you have to read a large paragraph of text below or on top of the photo to understand the picture -- in my eyes this is less a photo story but more a -- for sure also a good -- general story with illustrative photos where the text stands in the foreground; making a photo story I try to avoid this)About the scale of the photos I use 580 x 387 pixels for presentation. This results from my general homepage layout and the fixed photo relation of 3:2. I think this size (round about) is still big enough to see all details but also can be integrated quite well into a homepage or to be posted in a forum. Because we are speaking about a photo story, in case there is the need to focus on any detail within this photo you easily can add another photo with exactly this detail view, so there is no need for a higher resolution.
The original photos I always shot with the highest camera resolution (and afterwards scale it down to the presentation format). Big advantage with this is that you can crop quite easily or even focus on some details without re-shooting the photo again. And if you have to do some retouching this is quite esaier in higher resolution photos and becomes after rescaling more invisible than when doing so directly in the presentation format.
Apart from more disk space (what those days isn't really exepensive anymore) I see no disadvantage in using highest resolution.
Pesonally I do not like to scroll (in the sense of scrolling down a page) when reading a photo story; I just want to "turn a page" or at least have the single pics be arranged like in a comic book. So personally I like a gallery-styled presentation where each photo is presented on one page and by click you move to the next picture.
Anyway, in a forum environment such an presentation is actually a littel bit difficult.
Refering to what Tim_W said about posting comments inbetween still posting the photo story, maybe it might be an option to post photo stories in the PlaymoFriends gallery: opening for each single story a new folder; navigation between pics can be done via the navigation; you can add captions; photos will be scaled/shown in a reasonable size automatically; continuing the story is still possible by adding photos and no comments will inbetween. Presenting the story would be in that way, that you post your pics in the gallery unhurriedly just as you like and just open a thread with a reference/hyperlink to the story. So comments would be on one place separated from the story itself. Just an idea ...