There are various ways you can subscribe to this site for daily updates. The major two ways are via RSS or email.
If you prefer you can subscribe to this site via email. Each day you will get an email containing the daily picture. Just enter your email address in the box and follow the instructions.
RSS is basically a way of having a program check this website for updates. You run a RSS Reader and tell it to monitor the RSS Feed for this site and it will check every minute, 10 minutes, hour etc. It saves you having to come back to this site every day to check for updates. If there is an update the RSS Reader will tell you. This is the RSS Feed for this site.
Recommended RSS Readers
Aside from 3rd party RSS Readers most modern browsers and email clients can also subscribe to a RSS Feed.