Making an image draggable

[Douglas Gilliland]

I know it must be easy but I cannot figure out the script to make an
image of a metric ruler draggable so my students can drag it around
the screen to measure items in a quiz stack I am writing.
Tried several variations of the following script:
on mousedown
make image "ruler.jpg" draggable
end mousedown

Thanks for any help for this newbe.
Doug Gilliland