per channel activation

[eerie]

Should it be difficult to set 4chan.pl to only download images on selected channels?

[Christian Garbs]

If you do it yourself (in your local 4chan.pl script), it's rather easy.
In line 151 you can add an if() that checks $channel, $nick or whatever you like - if you don't want 4chan.pl to act, just return from there, eg if ($channel eq '#foo') { return }
https://github.com/mmitch/irssi-scripts/blob/master/4chan.pl#L151

Making this configurable is a lot more work to do - this means thinking of possible combinations to check (what does the average user want to do? what would the not-so-average user want to do?) and a means to edit this (invent a /command interface language for that), saving and restoring the settings...

If you just edit the code, Perl gives you all that you want.
To do it with a nice interface, I'd have to implement a subset of available Perl command in Perl.
I don't want to do that :-)

So my recommendation would be for you to fork the repo and make that one-line change to your copy.