I'm gonna assume that you can see the Debian box from XP. To get access to the Debian samba server, u would need a local user on that Debian box. If its a different username to what u are using in XP, when u double click on the samba share folder from XP, it will ask you for a username and password. So getting the user access setup is the first step.
Second, make sure that the folder you are tring to share has proper permission. Check the group and global permission on the folder.
Tha entry on smb.conf should look like below.
comment = Photo collection
path = /Pictures/
writable = yes
share modes = yes
force group = groupname
The masks setttings are upto u. and the bottom two are optional as well.
Hope it works for you.