You can't have a transparent profile avatar image in Slack in 2022-11-29 :(

This was requested in 2014, but did not change in 2017 and 2022: https://twitter.com/slackhq/status/533006882588622848

Manually set an image via curl (if you have a token with scope users.profile:write):

curl -s -XPOST https://slack.com/api/users.setPhoto --form token=xoxp-secret --form image=@myimage.png

while true; do echo -n .; convert cweiske-orange.png -fill blue -gravity Center -pointsize 130 -annotate +0+0 $(date +%H:%M:%S) -resize 128x128 temp1.png; curl -s -XPOST https://slack.com/api/users.setPhoto --form token=xoxp-secret --form image=@temp1.png > /dev/null; sleep 3; done