When I convert InDesign file to pdf, the background of my image is no longer transparent

I am working with InDesign CS3, and when I place a .gif file onto the document, the background of the image is transparent, which is what I want. However, when I convert the InDesign file to a pdf, that same image shows up in a white box. Why is this happening? Is there a setting that I am missing when I convert it? Thanks in advance