Comment 14 for bug 642417

> When you say everything is OK, do you mean Inkscape is OK?

The command you have tested is the one the extension uses in order to export your PNGs. Therefore if it works from the command line, it only means that Inkscape is in your path variable (which is not really a surprise since /usr/bin and /usr/local/bin are both almost always in the user path), and that it accepts the arguments used by the extension. Except if the arguments are not exactly the same...