If store by reference was specified with the Insert Picture menu option in order to insert the file, then
GetValue ( ContainerField ; 1 )
Will return the pixel dimensions of the image. (5000 x 6000 or some such)
If the file is NOT inserted with "store a reference", Length ( ContainerField ) returns the size of the file in bytes.
For files inserted with Insert Picture, GetValue ( Container field ; 3 ) returns the file path.
For files inserted with Insert File, GetValue ( ContainerField ; 2 ) returns the file path.