URGENT - size of image and fit on area

Hello
 
I’m creating a pdf file with images and text inisde
I would like to automate the resize of too big images….
My image could be bigger than the area containing it... so i would like
that xsl:fo fit my image and when the image is not bigger than the area
no resizing has to be done....
so as it’s not possible to specify the width and height of image because
it’s not generic.... how to do??
Use an external program to resize image with an automatic ratio?
Is it possible?
 
Help
seb