the file you're looking for is (tt-rss)/plugins/af_redditimgur (any other af plugin would do)
>So, I think is HOOK_ARTICLE_FILTER the hook I need, isn't it?
that's what i said
>Is this hook executed before ttrss_entries.content is stored in the data base?
well it would be rather useless to do it the other way around doesn't it
maybe start thinking a bit on your own before posting