You can change most of the images (for instance the logo, main banner on homepage, contents of participatory processes, etc) through the Administration panel.
As Decidim uses Webpacker, there is no way to override existing images including in Decidim.
If you want to add a new image you can do it as any Rails application, leaving it on your image’s asset folder (
app/packs/images) and using image_pack_tag helper (or asset_pack_path). You can see more documentation on Rails guide.