many thanks
If you knew up front that you needed this to happen, then yes, it's pretty
easy - you choose Link when you Insert, Picture, From File (the exact
procedure from there depends on the version of PPT you have - it's a
checkbox in PPT97, in PPT2000 and later, you click the down-arrow next to
the OK button and choose Link)
Then when you change the original file, the presentation updates itself to
the new graphic when you open it up.
If the presentation's already created, it's not so simple. There's no
automatic way to do it.
If the images are all the same size and in the same position, simply insert
the new picture, then copy/paste it to each of the other slides where it's
needed (after deleting the old one).
If the image belongs on every slide, put it on the Slide Master and your
job's done in one neat move.