Returns a thumbnail image of the content in a container field, according to specified values for width and height.
field - any text, number, date, time, timestamp, or container field; or any text expression or numeric expression.
width - the width for the thumbnail.
height - the height for the thumbnail.
Data type returned 
Originated in 
FileMaker Pro 12.0
Returns an image that’s stored in a container field according to specified values for width and height. The thumbnail image always maintains the proportions of the original image.
Note  If the field parameter does not specify a field that contains image data, field must evaluate to the file path of an image. See Creating file paths.
GetThumbnail(Dog;GetLayoutObjectAttribute("rectangle","width"); GetLayoutObjectAttribute("rectangle","height")) returns an image stored in the Dog field that fits into the dimensions of the Rectangle layout object.
GetThumbnail(Property;GetWidth(Property)/2;GetHeight(Property)/2)returns an image that is 50 percent of the size of the original image in the Property field.
GetThumbnail ( "image:question.png" ; 50 ; 50 ) returns a thumbnail of question.png with a maximum height and width of 50 points.
Related topics 
Functions reference (category list)
Functions reference (alphabetical list)
About formulas
About functions
Defining calculation fields
Using operators in formulas