The simplest possible solution (maybe) to use ruby expressions instead of awk, sed and all the other tools whose syntax you will never remember.
To rename all files named file1.png, file2.png and so on to vacation1.png, vacation2.png, first do this:
$ ls | rubyeval "'mv ' + f + ' ' + f.gsub(/file/, 'vacation')"
mv file1.png vacation1.png
mv file2.png vacation2.png
Verify the mv-commands. If they look ok, add the -x parameter (execute):
ls | rubyeval -x "'mv ' + f + ' ' + f.gsub(/file/, 'vacation')"
mv file1.png vacation1.png
mv file2.png vacation2.png
And that's it.