Try to get hold of the base64 encoded format of the image file attached, set the value to a clipboard page / data page property and use image control to display the image in your flow action section.
Thank you for reply.
I try above but I cannot display full size image.
Instead, I tried pzDisplayImageInline control and succeeded in displaying full size image.
I think pzDisplayImageInline control supports PegaSocial-Document but does not support Link-Attachment.