Goto to the directory:

cd ~/.thumbnails

And execute a find command with xargs:

find . -name '*.png' | xargs rm

Or you could:

echo * | xargs rm

How to create a 1000 files:

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