I believe this should be an Eggplant FAQ if not already. You must look for image elements that uniquely are available for any given OS.
(** assumes images to use for OS search are in the Suite's images folder
and each is properly tagged with a meaningful description field **)
set allowableTypes to ( tiff, icns, ico, pict, gif, bmp, png, pdf, jpg, jpeg )
set dirPath to my folder's folder & "Images/"
repeat with each item name in files of dirPath
if fileExtension(name) is not in allowableTypes then next repeat
if ImageFound(name) then return FoundImageInfo()'s Description
Throw "OS Detection Exception", "No OS was detected"