As far as I know, the set of properties returned by the Finder didn't changed for years.
set thePicture to ((path to desktop as text) & "The Great Wave.jpg") as alias
tell application "Finder" to set x to properties of thePicture
(*
{class:document file,
name:"The Great Wave.jpg",
index:73,
displayed name:"The Great Wave.jpg",
name extension:"jpg",
extension hidden:false,
container:folder "Desktop" of folder "yvankoenig" of folder "Users" of startup disk of application "Finder",
disk:startup disk of application "Finder",
position:{-1, -1},
desktop position:{1353, 438},
bounds:{-33, -33, 31, 31},
kind:"Image JPEG",
label index:0,
locked:false,
description:missing value,
comment:"",
size:1359930,
physical size:1363968,
creation date:date "jeudi 10 décembre 2009 04:12:19",
modification date:date "jeudi 10 décembre 2009 04:12:19",
icon:missing value,
owner:"yvankoenig",
group:"(inconnu)",
owner privileges:read write,
group privileges:read write,
everyones privileges:read only,
file type:missing value,
creator type:missing value,
stationery:false,
product version:"",
version:""}
*)
set theSize to size of x
--> 1359930
From my point of view it's more efficient to extract it with System Events
set thePicture to ((path to desktop as text) & "The Great Wave.jpg") as alias
tell application "System Events" to set the_size to size of disk item (thePicture as text)
—> 1359930
But, are you really wanting the size of the file or the dimensions of the picture ?