SMF does not allow cookie domains so the forum would operate on whatever one domain you have it on as and not use .digitalscrapbookpages.com so you need to make sure Photopost domain is the same as SMF
As in your smf is on digitalscrapbookpages.com and photopost on
www.digitalscrapbookpages.com with a cookie of .digitalscrapbookpages.com will not work also SMF will not recognize the cookie with .digitalscrapbookpages.com
Thats a young forum and they have not gotten around to allowing you to specify a cookie domain so your domain cookies must match exactly