You could queue up tweets - posting them slowly over time - not immediately once accepted - would that work? Or is the point to show people their contribution immediately with an attached image?
Rate limits for tweets are a bit more liberal than 15 every 15 min as far as i know - it’s more like 1 or 2 thousand in a 24 hour window (broken down into smaller windows of some unknown limit) https://support.twitter.com/articles/15364
Might be a good idea to add a phone to the account, start off tweeting manually maybe, get a few followers first, and then post automatically. Anti spam system seems to be strict on brand new accounts that immediately start posting lots of tweets from the API, without any other “non spammer-like” activity.