In SQL Developer 3.2.10.09 :
Make an explain plan of an sql request with a "SORT".
Export this explain to html code to a directory.
The HTML file is using a sortascending_ena.png image file but this file is missing in the images sub directory.
Not sure why that png (and lots of others too) is not getting copied into the images sub-directory of your user application data folder for SQL Developer. Probably each component is supposed to be responsible for checking that directory and copying images over there as needed.
I logged a bug: Bug 14770237 - EXPLAIN PLAN EXPORT HTML NOT COPYING ALL REQD PNG FILES TO IMAGES DIR
You can extract it from your installation's ...\sqldeveloper\sqldeveloper\extensions\oracle.sqldeveloper.jar file, from directory \oracle\dbtools\raptor\images using one of the unzip utilities.
SQL Developer Team