MS word 2003 with image PNG creates a pdf with image not selectable

I createted a rtf file with  word 2003.

In the rtf I inserted a image from file .png.

I printed rtf with pdf creator 1.2.3.

In the pdf created the image is not selectable to copy (I need!).

If I insert a jpg in the rtf file, in the pdf created I can select the image.

Can anyone help me?