Import your dxf file (File>Import>DXF).The procedure of creating a 'solid' logo from the DXF lines is a bit lengthy, but very reliable and gives high quality results. Might not work well with pictures, but since most Logos also come in 'simplified' vector forms, it might still be an option. If you have your logo in vector form, you should export it as dxf which Altium can import.
