overriding number of posts in “the loop”

Can the definition in the admin area for the number of posts to display be overridden? I am setting up my photoblog and I only want a post per page. In order to have the nav work (“previous_post()”), I have to set the number of posts to be displayed to be “1”. But in my archives, I’d like all of my thumbnails to show up on a single page. In order to get that to work, I have to set the number of posts to display in the admin to some number greater than the number of posts I have. I’d really like to have both work at the same time.
So, is it possible to override the number of posts to return in the loop? I’ve looked at the function and it doesn’t seem to take a parameter. Possibly a global variable? Also, is there some way to display ALL entries instead of faking it with a large number? Clearly, the first part is more important than the second.
Thanks.