Greetings. Recently posted to the slack app directory and were just featured earlier this week. Have you looked at other applications within the same category and contacted the founders of those apps? You might get some ideas and tips.

Also, check out the conferences and events

slack.com/events
slackfrontiers.com/
connectingyourtoolswithslack.… <---- i think this one will be good. I'll be there too =)

You should treat twitter like any other marketing channel. You'll need to test to determine if it's a good channel for your business. Can you provide more information on your product and customer?

For avatars, or what's this for?