参考URL

http://juribendini.tumblr.com/post/143545680586/mplayer-error-in-skin-config-file-on-line-6-png

内容
MPlayer - Error in skin config file on line 6: PNG read error in /usr/share/mplayer/skins/default/main

How to fix this error is described here. Just create a new bash file, with this content:

#!/bin/bash
cd /usr/share/mplayer/skins/default
for FILE in *.png
do sudo convert $FILE -define png:format=png24 $FILE
done

Save it in a file like conversion.sh and run it from terminal: bash conversion.sh
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同じ症状でしたが、これで起動しました。