Replacing last 3 characters of values
There are several ways to do that. 

The first one that Rick already mentioned will work on Postgres as well:

update genres
   set image = replace(image, '.png', '.jpg');


But that will not work if the file names contain .png before the end. To be sure to only replace that value at the end you can use a regular expression:

update genres
   set image = regexp_replace(image, '(.*)\.png$', '\1.jpg');


or you can simply use the first "length - 3" characters and append the new extension to that.

update genres
   set image = left(image, -3)||'png';


Alternatively as you seem to not like the || operator that has been defined for SQL 30 years ago:

update genres
   set image = concat(left(image, -3), 'png');