I think you want something like this (changed your joins to proper ANSI ones):
select
iu.UserName as UserNameWhoDisplayed,
pu.UserName as UserNameWhosePost,
sum(i.Count) as [Count]
from ItemDisplayCounts as i
inner join aspnet_Users as iu on iu.userid = i.whodisplayedid
inner join posts as p on p.postid = i.postid
inner join aspnet_Users as pu on pu.userid = p.whopostedid
group by
iu.UserName,
pu.UserName