Auto-populate image with text field

You can set a button field to display an image. Then all you need to do is create two buttons, one for each image, and then show/hide them based on the value of the text field.

You can use something like this as the text field's custom validation script to achieve it:

this.getField("AppleImage").display = (event.value=="A") ? display.visible : display.hidden;

this.getField("BananaImage").display = (event.value=="B") ? display.visible : display.hidden;