Whenever you want to leave a message to another player, but said player is offline at the moment, you have the option to leave them a book in-game. Of course, this assumes that the person will find and read said book, which is not a guarantee when everyone travels a lot!
A better option involves the forums: as every Creeper's Lab player has an account, you can look them up, send them a private message and wait for the answer. This makes it much easier to write longer messages and includes links and image, as you need. However, not everyone checks the forums, making this a gamble.
To improve on that situation, whenever you connect to any Creeper's Lab server and have a private forum message waiting for you, you'll get a notification after a few seconds!
When you get such a notification, don't forget to go check your messages - it might be something important! As a reminder, you can quickly look up a player directly from the website, using the "Player lookup:" search box under the main navigation menu.
This is the first of such notifications to be included, more will be added if there's a demand or need for it!