You can either create a new template in the Landing/Capture Page Tool by clicking the Suggest New Style Template (or you can view the style templates, and create a new one based on a current one), or you can add it to the appropriate area of your Landing/Capture page, or blogi (this is the easier option).
To add images to your header or sidebars on a landing or capture pagei, just click "Advanced Options" when creating or editing your landing or capture page. You will see an option to upload a logo for the header, enter various SEO meta tags, and also an area to edit the left and right sidebars. You have the option of using the image icon on your rich text editori to upload an image to those sidebars.
The same options are available for your blog. You can either use the advanced options for a particular blog post, or you can edit the settings of your entire blog so that every page of your blog (the blog, as well as all of the blog posts within that blog) has the same look and feel. Since you can have multiple blogs, each blog can have a different look.