We seem to get a higher open rate when we send in the evenings. Is it possible that because we have a larger number of people opening via mobile devices, that those images are automatically downloaded, and that we are getting a skewed open rate?
The iPhone Mail application will automatically load the email content, but from what we've found, it won't load the images until the email is actually opened, so that shouldn't be affecting your open rate on those devices. The criteria for a received email to be tracked as an 'open' is for either the images to be loaded, or a link to be clicked in the mailing. You can read more about how we track opens in our Resource Center article here.
Some email clients will automatically load images for their email preview, so those can be tracked as an open even if the recipient hasn't actually "opened" the message in their email client.
I hope this information helps, and don't hesitate to let us know if you need anything else!