You can change the company logo visible in left hand top corner of the community page from the admin page, in the "Company profile" section. The optimal size for your logo is 260x90 pixels and the recommended format is .png.
Community admins are able to send info messages to community members containing text, links, pictures and YouTube or Vimeo videos. The maximum width for attached pictures is 469px, wider pictures will be scaled down automatically. Admin user can send infomessages from web or from the application.