Goto to the directory:

<code>
cd ~/.thumbnails
</code>

And execute a find command with xargs:

<code>
find . -name '*.png' | xargs rm
</code>

Or you could:

<code>
echo * | xargs rm
</code>

How to create a 1000 files:

<code>
for (( i=1 ; $i<=1000 ; i=$i+1 )) ; do echo ffffffffffffffffffffffffffffffff > ffffffffffffffffffffffffffffffffffffffffffffffffffffffffff$i ; done
</code>