Why are the medium icons in the default skin just dots?

From TrillWiki

The true medium icons are copyright and trademarks of their respective owners. Cerulean Studios cannot use these without violating current copyright and trademark laws so after Trillian 1.0 they chose to use the new generic icons instead. During the beta test for 1.0, someone decided to make or modify the set from another skin so that they would work properly with the Whistler Skin. Trillian 3.0 has introduced the ability to use Icon packs to make this process easier, however, the specification for them isn't finalized yet.

For Trillian Pro 1.0 and the Whistler skin you can download this Zip file and install the original medium icons manually. Extract all of the files in the zip file to %trillian%\skins\Whistler\icons and when it prompts for you to overwrite the existing files answer yes. (Where %trillian% is where you have Trillian installed) Restart Trillian and enjoy the original icons. I would appreciate that you not redistribute the icon zip file to anyone else and that instead you would send the people here.

For Trillian 3.0+, search the forums for 'IconPacks' (such as Tomethicons). Most of them will include an installer or instructions on how to install. While Icon packs aren't finalized in Trillian itself yet, applications like The Pack Rat will allow you to change iconpacks back and forth.