Filename of an image

[Peter Haworth]

If I set the filename of an image, it correctly loads that file into the
image.

Say I then change the contents of the file - if I set the filename of the
image again, the original file contents remain in the image, presumably
because of some caching effect.

Is there a way to force the image to be reloaded from the file?  I tried
setting the filename to empty but that gave me an error (in the message
box).