Antetype screens can be exported as PNG, TIFF or vector-based PDF. Choose Export > Screens as Graphic… from the tool bar.
You can export multiple screens at once by selecting the check boxes at the bottom of the save dialog. The name of the resulting graphic will be equal to the screen name, but it can also be overwritten. Double-click the screen name in the save dialog and enter the new name. Existing files will be overwritten by default, but you can keep them by unchecking the "Overwrite files" checkbox.
For Retina exports special @2x and @3x checkboxes are provided. If checked, the normal and Retina resolution are exported.
In addition you can change the image size by changing the resolution.