Sorry for the very late answer, but I was able to test your problems only yesterday. I have tried to reproduce your bug, but without success :(
...well almost. Backslashes turn into forwardslashes, and there is always a character missing, so if I chose: C:\images\file.png
I will see: C:/mages/file.png
This used to work OK (within the past 12 months). I haven't had any need to insert any images of late (I already had several to trace), so I'm not sure when it stopped working.
I have tried on both XP and Vista Windows OS, but insert image worked without problems. Is there anybody else who also observes this problem? Are you able to try it on different machine? May be it is some local setting of your machine. Could you please try to uninstall your old therion and install new 5.2 version again? If it will not help - then I will have to write some debugging script to see, where exactly in the code this problem occurs. Regards, S.