There are two different approaches on how to post a Slack message with an image.
A. Upload image directly with message
You can upload an image directly to Slack and share it in a channel. Within that request you can also add a comment that will appear as message above the images. This is the easiest approach, however you comment is limited to one string.
API method: files.upload with these arguments:
channels: ID of one or multiple channel for the image to appear ininitial_comment: Your message
B. Post message with image block / attachment
Alternatively you can add an image to your normal message either as image block or secondary attachment. This only works with a public URL to your image file, so you first need to upload your image to an image hoster (which can be your Slack workspace) to get the public URL.
In our example we will use Slack as image hoster, but you can use any image hoster (e.g. Imgur) even your own webserver, as long as you get a public URL for your image file.
Step 1 - Upload image to Slack
API method: files.upload with no special arguments, but make sure to get the file ID from the response. Don't include the channels argument or the image will be posted visible into those channel.
Step 2 - Create public URL
Next you have to mark the uploaded file as public. Only then will it be accessible through its public_url property
API method: files.sharedPublicURL with the file ID as argument.
Next you need to construct the direct image link from the link to website / permalink_public property of the file.
The website link you get from permalink_public has the format:
https://slack-files.com/{team_id}-{file_id}-{pub_secret}
The direct link to the image has the format:
https://files.slack.com/files-pri/{team_id}-{file_id}/{filename}?pub_secret={pub_secret}
So you just need to extract the pub_secret from permalink_public and you should be able to construct the direct link to the image. The other parameters you can get from your file object.
Step 3 - Send message
Finally compose your message with the image URL either as Image Block or in a secondary attachment and submit it using a method of your choice.
API method: chat.PostMessage or any other method for sending message incl. incoming webhooks.
Answer to OP
If you need to stick with webhooks as it appears from your comments and have no access to the Slack API I would suggest uploading the image to an image hoster (e.g. Imgur) and then use approach B.
See also
