Canon PowerShot SD850 IS Digital ELPH User Manual page 42

You can shoot images with the optimal settings for postcards by
composing them inside the print area (width-to-height ratio of
approximately 3:2) shown in the LCD monitor.
For printing instructions, refer to the Direct Print User Guide.
Embedding the Date in the Image Data
You can embed the date in image data when [] (Postcard)
is selected.
4O