Need help with a SQL select

Hi.  I have a wallpaper program and I recently switched it to using a database to select images from rather than the file system.  I created a table called "wr_media" to store the info on the files themselves.  I also created a table "wr_tags" to store the tag data about each picture.  (wr = WallRotate, the name of my app).  Now I have a feature called "groups" in my app where I can specify a list of tags that I want it to pull from so if I make a group titled "Muscle Cars" I can put "camaro", "mustang", "chevelle" tags in the group and when that group is chosen, it only pulls wallpaper that matches the tags.

Problem I hit is actually implementing that.  I have a "test group" button that will bring back the total # of papers matching the tag criteria.  This is the SQL I tried:

SELECT COUNT(DISTINCT filename) AS papercount FROM wr_media INNER JOIN wr_tags ON wr_media.id = wr_tags.parent WHERE tag = 'camaro' OR tag = 'mustang'

Problem is, I get the same count when I remove the mustang tag, then I realized since I said "OR" that's probably the problem.  I switched to AND, but that fails because obviously a single tag field cannot be both values at once. 

Augh!  How would I write the SQL to pull back the records that have all required tags?  Media table is indexed on "id", and tags is linked by "parent" to media's "id".  Thanks!