For all the people still having trouble with this, here is the solution that worked for me. This is how I managed to get ImageFlow working with HighSlide onClick.
Open /wp-content/plugins/nextgen-imageflow/admin/install.php
Change line 16 with this
$ngg_if_options['ngg_if_onClick'] = "function() { return hs.expand(this,{ src: this.getAttribute('longdesc')}); }";
Open /wp-content/plugins/nextgen-imageflow/lib/functions.php
Change line 94 with this
$out .= "\n\t\t\t".', onClick: '. $ngg_if_options['ngg_if_onClick'] .'';
Now go to ImageFlow admin pages, click on “Setup” and then on “Reset settings”.
Now when you go back to General Settings OnClick behaviour should read
function() { return hs.expand(this,{ src: this.getAttribute('longdesc')}); }
which is correct (no extra \)
Hope that helps!