Make sure you remove the EXIF data (I use the command-line exiftool utility, but there are online equivalents you can use if you trust them) - sometimes it will save an old version of the thumbnail and sometimes erased info can be reconstructed from this, or your username (which maybe your email or other sensitive info can be guessed from) could also be in the EXIF).

I am curious why you picked the vector tool Inkscape, since it seems like you are marking up a raster image, which GIMP can certainly do.

-Arlo James Barnes

PostScript: In a thread related to this one, someone (I think Steve Litt) said that screenshots are always raster, which is true. But why? X.org simply describes the windows with some measurements, which could easily be translated into vector shapes, and this is true for window managers like GNOME3 and compositors like Compiz, too, right? Could there be a vector-screenshot capability (with embedded rasters for what cannot be represented as a vector) for future operating systems? This is pretty much off-topic for this list, which is why I hid in the the P.S., but it is an interesting thought.