Navigate to ADMIN > SITE > SITE LOGO.
The image assigned in your Admin area under the Site Logo field is used for the header logo.
You can type in the name of a media file you have previously uploaded, or you can upload a new image and it will save the logo in your media files.
It is recommended to use a .PNG or .JPG file for the logo. A .PNG file will allow for any transparency in your logo to show up.
Logo Size: ~ 204 px x 65 px
Resolution: 72 px/inch - Save for Web
Correctly size your logo before uploading it into the site.
Help Site - How To Resize And Crop Your Images Online