Image forming system forming specific image at set position

An image forming system comprising:

• an image forming portion configured to form an image on a sheet based on image data;
• a stack portion configured to store a stack of sheets on which images have been formed by the image forming portion;
• a specific image processing portion configured to cause the image forming portion to form a specific image on a specific sheet that is discharged to the stack portion before or after one or more sheets on which one or more images have been formed by the image forming portion based on the image data;
• a setting processing portion configured to set a specific image formation position on the specific sheet so that the specific image is formed at the specific image formation position;
• and
• a shift control portion configured to shift the specific sheet with respect to the sheets on the stack portion, wherein
• the setting processing portion is configured to set a plurality of specific image formation positions that are different from each other, in correspondence with a plurality of pieces of the image data,
• the setting processing portion further selects one of a plurality of specific images that are different from each other, as the specific image that is formed on the specific sheet,
• an amount by which the specific sheet is shifted with respect to the sheets is predetermined, and
• the setting processing portion sets the specific image formation position to be included in a region of the specific sheet that does not overlap with the sheets.