I don't know if you've seen this yet, but I've just posted this response in another discussion:
After much fiddling around in the tex filter code I found the problem: for some reason the line that runs the command to convert the .ps file to a .png (in my case) had become wrapped - i.e. had a carriage return in the middle of it. We've no idea how that happened.
Thanks for all your help Mauno . Everything is now behaving as it should.
Everything now seems to be behaving itself - thank goodness.