Find it under "BlogSite Options->Branding Options"
You can choose to use the site's Title and Description (editable under "Settings->General") or you can upload a custom logo image.
Each theme varies in size and placement of the logo. Both Latte and CuppaJoe have specific sizes to work with the design. A .jpg is fine in both cases. With Espresso and the CuppaJoe Blog Only versions, the logo sits on top of images so you will want to upload a .png file on a transparent background.
You can edit the title and description under "Settings->General Settings".