To add a logo go to site customisation, then click on General and then Logo. Two logos are present one is a general logo the other a retina logo for smart devices.

If you do not have a logo simply remove both, if you do have a logo click select image and then upload the desired image. Logos should be square and either Png. or Jpg. Format with a minimum size of 125x125 pixels. Retina logo should be twice the size.