add fish-key.png to blog files, change a link in the fish-key post accordingly

posts/fish-key.md

@@ -2,6 +2,6 @@
 
 My friend [Agata](http://aand.dkonto.pl/chmur/) had a good idea of creating a key for [12-tone equal music notation](http://musicnotation.org/systems/group/whole-step-5-lines/). It's called Fish key and it shows where F sharp (Fis in German naming) note is laying. Enjoy!
 
-<img src="/img/fis_h_key.png" alt="fis(h) key" style="height:200px;" />
+<img src="/files/blog/fish-key/fish-key.png" alt="fis(h) key" style="height:200px;" />
 
 I wrote about alternative music notation in a [past post](/blog/equal-music/).