Good news from the developer, Briask:
The module is safe, pretty much all it comprises of is some Javascript hooked into the Joomla module system.
That's a relief!
I don't know what changes Joomla has introduced in 2.5.x but it is highly likely that the 1.7.x ImageSlideShow will not work in 2.5.x . It should not be difficult for it to be upgraded to 2.5.x, you have all the code in the php&javascript files in the module download.
Well, it's working so far!
There are no updates available for the module.
Anyway, I wanted you to hear the outcome of this inquiry.