Sending Images and Documents in DMs and Channel Messages

You can use Markdown to link images, and they will unfurl in Slack.
You can use Markdown to link documents or images. This way Images will unfurl in Slack.
For this, just follow the Markdown format as shown below. You can also look this up for reference under the Message box (More Formatting link).
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If you don't have a link handy for image, you can upload an image through an upload site like imgbb​
Here's an example with an image.
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The above message will render as below in Slack.
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