Increasing use of images creates very large PowerPoint files. Many web hosters cap the size of e-mail attachments at around 10MB, a limit that is now very easy to exceed. Some suboptimal solutions:

• Upgrade to PowerPoint 2007, files are a lot smaller, but many of my (corporate) clients do not have this software yet, so you end up saving in the 2003 format anyway
• Compress pictures: select the picture, in the format menu pick "compress" and select the appropriate DPI rate. I personally don't use it a lot: it does not save that much space if you use larger images, and the quality of your source file deteriorates forever. A big issue, especially when putting your presentation on a big overhead screen.
• Zipping files does not have a big impact when using images

What I end up doing is

• Keep file sizes (and image quality) to the maximum
• Use PDF files to exchange drafts with my clients
• Finally, send the master file across using a file transfer utility such as YouSendIt.com (note that YouSendIt is not secure in its basic version, anyone "guessing" the URL can download the file).