To add your logo to your share maps first go to your Profile (this is located in the top right next to your Dashboard button). Choose to Edit Profile and then click on the photo upload at the bottom of your profile edit. Browse your computer for your logo (do not use a PDF) and upload it to your profile. Once you have uploaded your logo click Update Profile. You should now see your logo in your profile and in the top left of all your share maps.
See video below for a visual example.