Chatting
Starting a conversation
Simply double-click a contact and a new window/tab will open with their conversation. Then you can start chatting:

Sending a screen capture
To send a screen capture, the recipient must be on a service
that Trillian is capable of sending images through. At this time,
Trillian supports sending images to other Trillian
Network users. What a great reason to get your friends
using Trillian!
From your message window click Insert > Screen Capture...

Then drag a box on your screen with your mouse:

And your screen capture will appear in your edit box! You can now continue to type or just send it plain.

Tip: Ctrl-V will paste any image from your clipboard. Tip: Copy your entire desktop to the clipboard by pressing your PrntScrn key; or, use Alt-PrntScrn to copy the active window. Paste into a chat with Ctrl-V!
Quick Reply
Reply right from message popup notifications!

Tip: If you are not getting notifications for new messages, check the box at Preferences > Notifications > (Notify me when) A new message arrives in a chat room window.
Timestamps
Press the F2 key on your keyboard to toggle timestamps in your current message window. The permanent setting is in Preferences > Chat > Chapters and Timestamps section.
Changing Message Styles
You can change the Message Style of your message windows between Simple and Bubbles from the More menu in your chat window:

Tip: You can now download more message styles to further customize your Trillian chat windows!