You can change most of the images (for instance the logo, main banner on homepage, contents of participatory processes, etc) through the Administration panel.

If you want to override the standard images given by Decidim (for instance the initial image for user’s avatars) you can do it by naming it the same way as the original image, in this case it should be app/assets/images/decidim/default-avatar.svg

If you want to add a new image you can do it as any Rails application, leaving it on your image’s asset folder and using image_tag helper. You can see more documentation on Rails guide.