Applescript & MS Powerpoint 2004 problem

I’m trying to create a simple script that will let me choose a folder of images and then create a quick powerpoint slide show of them:

set this_folder to (choose folder with prompt "Pick the folder containing the images to process:")
tell application "System Events"
	set these_files to every file of folder this_folder whose name does not start with "." and (file type is "TIFF" or file type is "JPEG" or name extension is "tiff" or name extension is "tif" or name extension is "jpeg" or name extension is "jpg")
end tell
tell application "Microsoft PowerPoint"
	activate
	set this_presentation to make new presentation
	repeat with i from 1 to the count of these_files
		set this_file to (item i of these_files as string)
		set this_slide to make new slide at the end of the active presentation with properties {layout:slide layout large object}
		-- copy the position and dimensions of the placeholder box
		tell shape 1 of this_slide
			copy {top, left position, width, height} to {shape_top, shape_left, shape_width, shape_height}
			delete
		end tell
		-- make a new picture box
		-- display dialog (this_file as string)
		set picture_shape to make new picture at this_slide with properties {top:shape_top, left position:shape_left, lock aspect ratio:true, file name:this_file}
		-- scale the picture to a smaller size
		scale height picture_shape factor 0.25 scale scale from top left with relative to original size
		scale width picture_shape factor 0.25 scale scale from top left with relative to original size
		-- get the dimensions of the picture
		tell picture_shape
			copy {width, height} to {picture_width, picture_height}
		end tell
		-- adjust the picture to fit
		if the picture_height is greater than the picture_width then
			set the height of the picture_shape to the shape_height
		else
			set the width of the picture_shape to the shape_width
		end if
		-- show the slide
		set the slide of the view of document window 1 to this_slide
	end repeat
end tell 

The problem is when it actualy gets to the make new picture line it fails with an error from Powerpoint saying :
“Microsoft PowerPoint got an error: Can’t make a picture.”

Any clues?

Thanks for the help

John Scherer