Auto write permissions?

PunnBB seems to me (in regards to the avatars, or the file they are contained in) to automatically write the avatars to be CHMODed to be readable/writable.  Is this so, or do I just not grasp the idea of file permissions?  The reason I ask is, one:  I plan on eventually (after I do ALOT more learning) to write my own webcomic/forum script; and two: because I use another script for my webcomic that I have to manually go and give the comic pages them selves certain file permissions to be able to be called by the script, whereas PunBB doesnt require me to do this for the avatars.  (I ran phpBB for a while {which I hated} and I had to turn off the "upload avatar" option because they wouldn't have the correct permissions.)  Or could it possibly be senario #3, which is, my host has changed the way in which it allows its users to run php?
Whatever the case may be, thanks for lookin' here.

(PS, if it is in fact that PunBB writes the avatars you upload with the correct permissions, could you reference me to which file and line {and an example} to where I can find it?)

Thanks!