Can you have more than four background frames for background animations?
Read the Manual (http://www.adventuregamestudio.co.uk/manual/Animating%20background%20scenes.htm), the BFAQ (http://americangirlscouts.org/agswiki/Graphics%2C_Characters%2C_Text_%26_Rooms#Animating_your_background_images), or search the forums (http://www.adventuregamestudio.co.uk/yabb/index.php?topic=21723.0).
The limit is 5 frames (the 'main' one you see in the room editor, plus the four you import in the 'Animate Background' popup). If you need longer animations than that, you'll have to use Objects (or dummy Characters, since you can move them from room to room to save Objects).