What image do you intend to display if someone enters a value into the combo box that doesn't have a corresponding image - or did you mean a select list rather than a combo box.
Combo Box (where someone can select an option from the list or type their own):
Code:
<input type="text" list="fruit">
<datalist id="fruit">
<option value="Red Apple">Red Apple</option>
<option value="Green Apple">Green Apple</option>
</datalist>
Select List:
Code:
<select>
<option value="Red Apple">Red Apple</option>
<option value="Green Apple">Green Apple</option>
</select>
Two completely different field types (although Andrew's JavaScript would work for both apart from needing an extra else for a combobox to cater for when the person enters "Purple Pears" or "Yellow Bananas" or "qwertyuiop".