Hi Prisca…first, if this is related to a specific theme, please post it in the appropriate forum category; so with that in mind, what theme is this for?
Now the bad news is that your questions relate to performing “customization” which support does not cover, but what I would suggest is setup a development site with the theme, and start modifying the code to achieve the results you want. I can tell you that moving a menu container to be below a logo will require you to have coding skills because you will need to move html, php code, and also modify css. Changing and or adding styling to images will also require you to have some html and css basics of understanding as well. Your other option would be to find someone who might be able to make these kind of changes.