Adding logos to all your slides is easy if you work with the Slide Master.
On the View tab, in the Master Views group, click Slide Master. You will likely want to position the logo differently on the Title Slide than on the remaining layouts. Insert the picture on the first layout, then copy and paste it to the second layout. Resize and/or reposition as needed. You can copy and paste that to the remaining layouts where you would like the logo. Leave some layouts without a logo in case you have a large graphic that might chop it off.