Thanks for your answer. I have used this size on several other games with no problem. I guess this time it must have to do with the way it is used.
I do educational games.
This game has math questions. The user goes to a random place in a mall. The stage/window is 600 x 400 and the long image has 9 shops (600 x 9 = 5400) . The questions are randomized and the image slides to a different shop/position depending on the question. There is a visual effect seeing the shops slide by in either direction depending on the question. I use liteTween to slide the image.
So I cannot use the tiled background because of it. I guess I could cut the image into 9 pieces and pin them to a container. I'll try that.
There are lots of other things going on too since the user has to choose several things and input text in boxes, etc...