Help with CS5 please!!!?

I am making a collage and I set the background to 12x18. I put my images in it, flattened and made it a jpeg saving it at the max quality. Then I went to mpix to order it and it says the resolution is too low and the largest I can do is 8x12... but the images can go much bigger than 12x18 by themselves. Does anyone know why this happened? I am new at making the collages! Thank you :)