did you happen to use the Tool command in Screenshot Captor to launch and edit the image file with an external program?
what format is the file?
The background was a JPG file, the image I added in the foreground (using "Edit/Copy image to clipboard" and "Edit/Past clipboard image as new object") PNG.
And what I noticed just now:
filesize of original JGP: 160 kb
filesize of original PNG: 73 kb
filesize of resulting PNG: 1342 kb!?