You can user Graph chatMessageHostedContent API to retrieve hosted content for message in a channel
GET /teams/{team-id}/channels/{channel-id}/messages/{message-id}/hostedContents/{hosted-content-id}
GET /teams/{team-id}/channels/{channel-id}/messages/{message-id}/replies/{reply-id}/hostedContents/{hosted-content-id}
To get hosted content bytes for an image, use $value as below sample request
GET https://graph.microsoft.com/v1.0/chats/19:2da4c29f6d7041eca70b638b43d45437@thread.v2/messages/1615971548136/hostedContents/aWQ9eF8wLXd1..V3cy9pbWdv/$value
you can find more details here https://learn.microsoft.com/en-us/graph/api/chatmessagehostedcontent-get?view=graph-rest-1.0&tabs=http#example-2-get-hosted-content-bytes-for-an-image
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