heavy dialogue is the bane of my existence... though honestly, light dialogue is probably just as much of a pain. much like in comics, dialogue is as much of an artform as your pictures are. if you neglect the dialogue, or treat it as if it's just there, your story will not flow well.
it's hard to make sure your dialogue and pictures work well together. eventually it kind of comes naturally, but in the meantime it can take a lot of practice. in a way, you can think of it like dubbing a foreign movie. too much dialogue can give the effect of a lengthy monologue while the mouth is moving enough for two words, or somehow the hero can squeeze in a lengthy rebuttal in the time it takes to throw a single punch.
one rule of thumb i use is to avoid making the dialogue longer than the width of the pictures. this of course can kind of vary depending on where you're posting your photostories (since different sites use different fonts and font sizes) but if you aim for just one line of dialogue without breaking to a second line, that's generally best.
but what about the times when you just have a LOT to say? this is where filler pictures come in. you can take a pic of random things, or have the character(s) talking in the background, or a shot of their hands doing something, etc. and extend your dialogue along these pics. (example 1 and example 2 -- #2 is NWS)
in the case of filler pics, the unimportant pics with nothing really going on in them helps balance the heavy dialogue, because the reader can spend less time focusing on the pic and more time reading without missing anything. in general, though, if you have more dialogue you want more going on in your pic (and likewise, less dialogue would have less going on in the pic). this is not always the case, obviously. you can use it to your advantage, to manipulate the flow of your story. for example, you would have a pic of just somebody's face looking surprised, with the dialogue of "WHAT?!" each picture has its own rhythm and your dialogue should fit without throwing in some extra drumbeats.
then there is something else i want to quickly address... it's not necessarily the same thing, but it does have to do with dialogue bogging down the flow of a story. often you'll see photostory writers include multiple lines of dialogue from the same person (or people) on just one pic. i'd advise against this, since, like i said, it bogs down the flow considerably. limit each character to one line of dialogue per pic, and stick to only letting 1-3 characters talk at a time. i've found this to be the best, because any more lines and readers are spending too much time focusing on the dialogue.
you want to find a happy balance. a balance between time spent looking at your pic and time spent reading your dialogue. a balance between too much talking and not enough talking. proper balance leads to proper story flow, like a beautiful melody or well-timed voiceovers.
(these tips have not been in any particular order, incase you were wondering. the numbers don't really indicate any level of importance.)