Hi,
We have the option to include images that appear as tooltips when the user puts the mouse over the codelist option. This only works when the code item is using the "Textbox" option.
When you edit a code item you can add the image (with the limitation of it being under 300KB)

Then in the Collect Earth balloon in Google Earth it appears like this (ATTENTION: there is a problem in the preview in the Survey Designer and the image is not shown properly there, only when you open the balloon inside Google Earth)

Maybe this is not the best way to show the possible representations of the different classes, so it might be better if you develop some sort of "Visual Interpretation Guidelines" document for your project that the interpreters can follow.