Keep it simple and visual
Focus on viz types that are very visual and easy to digest. Make sure the most important information is standing out even when people just scroll through their new messages. Therefore, select a few, large visualizations over complex dashboards with too much information. Ideally, you can extract the key message of the chart without enlarging the picture to fullscreen in Slack.
Consider the Default View
Design for a the default view (i.e. before setting any custom filters and parameters). While you can send filtered reports to Slack via Reports, the default filter settings will be what people see if they use the /pm command. Make sure this view already has some relevant information.
Provide Filters to dig deeper
While the default view should contain valuable information, the live view in Tableau is just one click away since the View Name above the image links to your Tableau Server or Tableau Online. Therefore have useful filters on your dashboard that allows people to dig deeper if they spotted something interesting.
Use Dashboard Sizes that fit for Slack
Slack will resize large images to a maximum width of 400px or a maximum height of 500px, while still maintaining the original aspect ratio. Therefore, specify Dashboard sizes that look good in Slack (and consider mobile usage). Generally, portrait ratios work well and since you can enlarge the preview image to zoom into details, the image might be much larger than 400x500.