We are using the Metabase Metabot to post “pulses” to a Slack channel, however, we found that our “multi-channel guests” in Slack are unable to view the images that are posted by the Metabot. They can see that something was posted by Metabot but the actual image is blank and shows as a broken link when clicking the URL of the image. Has anyone experienced this and may know what to do?
No, I have not experienced that - but then we don’t have multi channel guests (I just learned about those now from your post) and I only recently started toying with setting up MetaBot on Slack 
But, I might have a clue: If those guests don’t have access to the metabase_files channel where MetaBot is posting the images that might be the reason why they don’t see the images.
Setup of the metabase_files channel and why it is needed is described near the end of this page in the admin guide