Unless you set a profile picture for a team, the team profile picture will match the organization's profile picture.
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In the upper-right corner of GitHub, select your profile photo, then click Your organizations.
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Click the name of your organization.
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Under your organization name, click Teams.
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Click the name of the team.
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At the top of the team page, click Settings.
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Under "Profile picture", click Upload new picture, then select your desired profile picture.
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Click and drag to crop the image as needed, then click Set new team avatar.