Adding council logo can be done by following the steps in this guide
Same steps in more detail:
1) Create a copy of the analyst\theme\branding\default folder and rename to any brand name you wish. In this example, we have used ‘council’

2) Create a copy of the analyst\theme\banner\default folder and rename. Note that this step is optional and you could just edit the contents in the default folder itself.

3) Edit the file brand.css in \theme\branding\council folder (from step 1) and specify new banner file. Here it's pointed to the ‘theme/banner/council’ folder (from step 2).
Remove the /* and */ bits as they comment out the contents

4) Place your logo image file in \theme\banner\council folder. Edit the banner.css file in same folder and rename the #productlogo to your council logo

5) Lastly, point the map configuration to this brand, via SSA admin console
Same steps in more detail:
1) Create a copy of the analyst\theme\branding\default folder and rename to any brand name you wish. In this example, we have used ‘council’
2) Create a copy of the analyst\theme\banner\default folder and rename. Note that this step is optional and you could just edit the contents in the default folder itself.
3) Edit the file brand.css in \theme\branding\council folder (from step 1) and specify new banner file. Here it's pointed to the ‘theme/banner/council’ folder (from step 2).
Remove the /* and */ bits as they comment out the contents
4) Place your logo image file in \theme\banner\council folder. Edit the banner.css file in same folder and rename the #productlogo to your council logo
5) Lastly, point the map configuration to this brand, via SSA admin console
Environment Details
UPDATED:
November 30, 2017