First you will want to ensure the Image has a Transparent background. Here are some instructions on how to do this in photo shop http://kb.restaurantengine.com/remove-logo-background-make-it-transparent-using-photoshop/.

Once you have saved the Image with a Transparent background, you will be able to insert the Image into the Pushpage via https://www.blackbaud.com/files/support/howto/coveo/k12/bb-onmessage-insert-images-files.html