Hi Daniela,
We do have a Discourse Gifs theme component that you can use to add a “GIF” button to the post composer on a site. This would be the best option for easily inserting gifs into posts on your site using the composer.
I’m not aware of any theme components or plugins that would add a similar menu to the composer that would allow you to insert your own custom photos or gifs.
You might be able to use the Watched Words Link settings to replace specific words on your site with links to gifs or pictures, however, with this option the words replaced with links would typically just link to the content rather than replacing the text with the content.
I hope that helps point you in the right direction for what you’re looking to accomplish here. 