Have you compressed the pictures inside of Powerpoint?
If not, select any of the pictures in your slideshow and then find the
'Compress Pictures' function. This is different depending on your version,
but in Powerpoint 2007 select the 'Format' Ribbon and on your left in the
'Adjust' Section is the 'Compress Pictures' Button. This should help to bring
the size down.
If you finding its still to big, you have two options I can think of.
1. Resize the pictures outside of powerpoint using a photo editing program
like Irfan View. This will reduce the size of the presentation but also the
quality of your images.
2. Depending on what software you use for archiving, when zipping the
powerpoint select to Split the archive into different 'volumes'. You could
then send each smaller file as a seperate email. The receiver will then need
to download all of the attachments and the Zip program should take care of
putting all the pieces back together.
Hope this helps.