I am looking for our website's logo image and I seem not to find it on our CMS. Where should it usually be, or how can I actually look for it in the system?
I'm afraid this will depend on how the site has been built as you're not restricted to where you edit a given piece of content such as a logo.
If the site is based on Alloy, the logo can be edited from the site settings tab. For other sites it would typically be alongside the site settings either on the start page or on a dedicated settings page, and for others it may not be editable in the CMS at all (i.e. hard coded in the views or CSS).
If you want to find out for sure you'll need to look at your layouts/views where the logo is added and see how it's being added.
As Paul indicates, the only way to know for sure is to
Hope that helps