Custom fields are the simplest way to go, you don't need any mod.
In the directory /Themes/default/images create a new directory and call it "mood", place all the images you need in this directory.
Each file should be called by the mood state it represents, for example: happy.png, sad.png, etc.
Create a new custom field with this options:
* Show on Topic View => yes
* Show Enclosed Within Text => <img src="{DEFAULT_IMAGES_URL}/mood/{INPUT}.png" alt="mood {INPUT}" />* Field type => Select Box
* Put the mood states one per box (happy, sad, etc.)
Done.