Which email app are you using? Open the app, tap Menu>Settings, select the email account, and see if there is an option to automatically download images.
If you are using MailDroid, the images are blocked by default unless you have it to always show images in the settings. Most apps also do this because of security. In MailDroid a little circle icon appears with a bar at the top of the email saying images are blocked and clicking on that icon will allow you to always show images for that email address or allow it one time only.