My French is a little rusty but it seems to be saying that the image type has been rejected by the server and to choose another image or contact the site owner.
This seems to be a server configuration issue, so you will need to ensure all the relevant configuration and modules are installed.
Which file type are you trying to upload?
Does it happen with all types: .png, .gif, .jpg?
Browse to admin.php?tools/phpinfo and check which image types are supported under GD or ImageMagick.
Also, double check the server meets the requirement: XenForo System Requirements