Slack: How To Change Your Profile Picture

To do so, select your profile picture in the top-right corner, then select Edit profile.

press your profile picture in the top-right corner, then press Edit profile.

Tip: Images must be either a JPG, PNG or GIF.

Click Upload an Image, select an image from your rig, then crop it to fit.

I hope this article helps you to change your profile picture in Slack.