You must resize photo images for attaching to emails for two reasons.
1. To make the file smaller and so not clog up a persons email. Aim for 100 to 200 kb
2. To make the photo appear on the screen at a useful size. An average computer screen is around 1200 pixels wide - so if you resized an image to a width of 1200 pixels it would be too large and fill the entire screen. A good width to aim fo is 500 pixels which will show up as just under half the width od the screen and be larger emough to see clearly.
ONLINE SHARING SYSTEMS
These are normally free unless you want to hold hundreds of photos, when there will be a small fee.
www.flickr.com has developed as the most popular site for holding and sharing photos. Click on how to share photos using flickr to see an extract from the book Digital Cameras the easy way showing how to set up a flickr account and upload your photos. And try flickr Wikipedia for more information.
Picasa is another popular system, now owned by Google. The Picasa Wikipedia article shows how it works.