Emails are counted as opened when the images are downloaded from our server, which is the industry standard for tracking emails. This means that if the images are downloaded in a preview pane (or viewed in entirety in the whole campaign), then this will count as an open.
If someone has images blocked, that will not be counted as an open. In that case, those people would be counted only if they either loaded their images or clicked on a link in the campaign.
And if someone views the plaintext-only version of your campaign (for example, from a Blackberry), it'll only be counted as opened if they visit a link in the campaign.
The industry average open rate is about 20-25%. The Email Stat Center provides stats on all kinds of response averages.