The icons at https://arcabc.ca/islandora/object/bceln:root come from the TN (thumbnail) datastream on each site's root collection object. If you want to change that icon, you must manage your root collection.
Go to your site's root (at /islandora/object/[yournamespace]:root) and click Manage. Then click Datastreams, and under the TN datastream, click Replace.
Upload your new thumbnail. Make sure it is a 128x128 pixel square -- if its dimensions are not square, you will need to edit the image so that they are. Otherwise it will not display attractively in the list.
If the Partner Organizations page also requires an update to the icon, please let the Admin Centre know in an email, and attach your new thumbnail.