I do dotted borders on Gimp.
First, open the image you want to border ---> Select All ---> Edit ---> Stroke Selection ---> Line Width 1.0 or 2.0 ---> Line Style and Dash Preset to select the type of border you want, in this case Normal Dots, Dense Dots or Stipples ---> Stroke ---> Select None ---> Save the image as a .png or .jpg