Impossible? Sort On Dynamic Field (in Combination With Row_number)

I have images on which users may comment. Now I want to sort those images on the amount of comments each image has.
I use custom paging in combination with ROW_NUMBER
So I want to sort items in one table based on the number of occurences of that item in another table...

I have the following query (which doesnt work):

SELECT *
FROM
(

select ROW_NUMBER() OVER (ORDER BY mediaComments DESC) as RowNum,
m.title,m.usercode,m.mediaid,
mediaComments=(select count(*) from tblMediaComments where mediaid=m.mediaid)
FROM tblAlbums a
inner join tblMedia m on am.mediaid=m.mediaid
left join tblMediaComments mc on mc.mediaid=m.mediaid
WHERE a.showTo=1
group by  m.title,m.usercode,m.mediaid
) as Info


WHERE RowNum between @startRowIndex AND (@startRowIndex + @maximumRows-1)