The Ubuntu Visual Identity guidelines and assets are available on the Ubuntu Brand Guidelines website and its Downloads section. Please refer to the trademark policy for information regarding permission and correct usage of the logo.
Official artwork for Kubuntu is available here on the KubuntuArtwork page.
In .svg format (converts well to other formats):
Official artwork for Xubuntu is available here on the Xubuntu/Artwork page.
Official artwork for Lubuntu is available here on the Lubuntu/Artwork page.
Ubuntu Studio Artwork
Official artwork for UbuntuStudio is available on the UbuntuStudio/Artwork page.
Official artwork for Ubuntu Gnome is available on the UbuntuGNOME/Artwork page.