Images are cached based on the URL, so if you want to force an update, the easiest way is to add a random value to the querystring, so the URL changes.
Pseudo-example...
date = CreateObject("roDateTime")
...
url: "http://my.images.com/image?ID_NOTATION=324977&HEIGHT=75&WIDTH=120&TIME_SPAN=1&nocache=" + date.AsSeconds().ToStr()
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