I have had problems in the past finding royalty free clip art that clearly depicted the different types of heat transfer I teach in my 6th grade science class. Instead of searching, I created my own to use in SMARTboard activities, handouts, and PowerPoint presentations.
The 21 pieces of clip art included in this compressed folder depict different examples of how heat is transferred through convection, conduction, and radiation. Each has a labeled and unlabeled version of the heat transfer to use as best fits. All of the images are presented on a white background in a .png file format for easy inclusion into documents and activities.