Sometimes you may want to use one large image in an email. You can run into problems with Outlook if you use this approach.
If the image in use is over 1728 pixels high, Outlook will crop 250 pixels off of the top of it. This can produce very undesirable effects, as this is the region where company names, logos and other important information is often displayed in such images.
In cases like this, it would be best to cut your image up into several smaller ones and stack them on top of each other.