Executing click at on an app


I am trying to select some pictures in an app that has a NSCollectionView. To do that I started by creating a small script that gets the position and size of the app’s window.
I then tell system events to click random points inside a bounding box defined by the origin and size of that window. I should be getting feedback on the clicks by seeing the element selected, but I don’t see anything. How do I know that I am really clicking ?
Below is the script that can be adapted to another app.

tell application "System Events" to tell process "XekMyPic"
	set client_size to get size of window 1
	set width to item 1 of client_size
	set height to item 2 of client_size
	set client_pos to get position of window 1
	set wnd_xpos to item 1 of client_pos
	set wnd_ypos to item 2 of client_pos
end tell

tell application "System Events"
	repeat 10 times
		set xposition to wnd_xpos + (random number from 43 to width - 55)
		set yposition to wnd_ypos + (random number from 305 to height - 400)
		set xposstr to xposition as string
		set yposstr to yposition as string
		display dialog xposstr & ", " & yposstr
		click at {xposition, yposition}
		delay 3
	end repeat
end tell


In my UI scripts I always add

tell application process "XekMyPic"

end tell

around any clicking, etc. I also add an “activate” just before the UI interaction in case one of those users brought another app to the front.