Clerk supports sending and receiving inbound and outbound MMS (media messages) right in Slack both on desktop and mobile apps.
Inside of your Clerk SMS thread click on the Attach File.
Slack UI sometimes hides the Attach File button. If you see the screen below, click the blue arrow pointing left to see the full Slack menu.
To send an image from your Slack mobile thread click on Attach File.
Clerk offers support for common image file types: png, jpeg, and gif.
Maximum file sizes for MMS for the major carriers in the US