J'imagine que tu as lu la doc ! mais je te joins le principal des recommandations
To add a logo above your menu in Imagevue X2.8, simply go to admin › settings › menu.mainmenu.menu_logo, and add the path to your logo file. We recommend uploading your logo to the iv-config/ folder so that it gets stored safely even after upgrading Imagevue.
Save your logo in PNG 24 format with transparency enabled for best results.
You can use any dimensions, and the menu will automatically adapt. However, we recommend a width smaller than 200 px and a height 50-200 px depending on your logo aspect.
Upload your logo to the iv-config/ folder by means of FTP.
Your path in the menu_logo setting should be something like: iv-config/mylogo.png.
You can download an example of a logo in PNG 24 format here.
Bon courage à toi !