hiya
I'm not so sure you can resize pics that are on the 'email server side' at present. I may be wrong, but as yet I haven't specifically come across any software or ISP functionality that does this. The pic files are large and are currently 'at' the ISP email server (or Yahoo or hotmail's servers) It may be worthwhile speaking to the tec support of your email provider to see if they offer this functionality.
If not, you 1. could ask your dad to resize them and re-send or, bite the bullet, download all the large pics to your PC and then resize them. If you visit Microsoft.com and search for Powertoys for XP (assuming you're on XP, if not just search for Powertoys) and you'll see a number of enhancements. I currently do use the 'image resizer' - download and install that. Once installed, it's easy to then right click on a large image, and you'll see some sizing options and a custom setting. Choose a lower spec and the software will resize the image for you. You can choose the 'resize original do not make copies' option if you'd like. However, as it's family photo's, I'd suggest making copies and keeping the high quality originals alone. That way, if you should ever choose to print or make larger for poster sized prints, you'll have the most amount of 'quality'
hope this helps!
Dennis! West London & Slough UK!