I'm not sure I'll do it all in one clip, but likely one chapter at a time. I'm leaning towards it's own thread due to my own log format for this. My thought is to create a bunch of "placeholder" posts in succession, then fill them in as the run continues. It's easy to plot out if I have an idea of a post's maximum limit.
Yes, there is a character limit per post,
@Antimatter can tell you how much.
And while sometimes one person or another who have been following a thread closely have made friendly requests of putting pictures in spoiler tags, because it affects their loading times, it is not a requirement.
Sometimes a post just looks better when some screenshots are visible. Of course if someone, like the no-reloaders often do, documents the tactical steps of a single fight with 15 screenshots in a row, spoiler tags could help.
You don't need to convert png to jpg either, unless it is to reduce the file size to get below the 3MB per picture limit.
I wouldn't recommend using placeholder posts and filling them in, because the forum sorts the threads by recently posted and that's how most people check for new content. If you create a thread with 20 placeholder posts and then start filling them in one by one with edits over time, your thread won't be showing up in the queue as having been updated recently and you won't get any views.