ImageJ community members are often available via a couple of different chat services.
The Gitter chat service, while relatively new, is a great way to discuss issues with developers of the SciJava software stack. All you need is a GitHub account. Advantages of Gitter include a nice "21st century" web interface, email notification when participants miss messages, and a full searchable history.
|#imagejdev||Core ImageJ development discussion.||imagejdev logs|
|#fiji-devel||Fiji user and developer discussion and questions.||fiji-devel logs|
Advantages of IRC include anonymity, and maximum compatibility with a huge variety of existing chat clients.