Mencoder Error

I am trying to merge several '*.png' files into a movie using mencoder. At present, I am using running the following on the terminal.

mencoder mf://*.png -mf fps=10:type=png -ovc x264 -x264encopts bitrate=1200:threads=2 -o movie.mkv

I keep getting an error message

import: unable to read X window image Resource temporarily unavailable @ xwindow.c/XImportImage/5020.

and after more modifications, this error:

ODML: Aspect information not (yet?) available or unspecified, not writing vprp header.

Can someone show me how to make this merging? Or where the problem is?

mf://*.png will expand to mf://first.png second.png third.png

What does mf:// mean anyyway?

Scratch that, it's even worse than I thought. mf://*.png won't expand at all, because there's no filenames beginning with mf:// in your folder.