Images can be added to options.
Example
<select class="form-control" data-onload-select2='{"minimumResultsForSearch": "Infinity"}'>
<option>No image</option>
<option
selected
data-image-src="//satyr.io/32x16?flag=cze"
data-image-srcset="//satyr.io/32x16?flag=cze, //satyr.io/64x32?flag=cze 2x"
data-image-width="32"
data-image-height="16"
>Czech</option>
<option
data-image-src="//satyr.io/32x16?flag=can"
data-image-srcset="//satyr.io/32x16?flag=can, //satyr.io/64x32?flag=can 2x"
data-image-width="32"
data-image-height="16"
>Canadian</option>
<option
data-image-src="//satyr.io/32x16?flag=che"
data-image-srcset="//satyr.io/32x16?flag=che, //satyr.io/64x32?flag=che 2x"
data-image-width="32"
data-image-height="16"
>Swiss</option>
<option
data-image-src="//satyr.io/60x60?flag=che"
data-image-srcset="//satyr.io/60x60?flag=che, //satyr.io/120x120?flag=che 2x"
data-image-width="60"
data-image-height="60"
>Swiss bigger</option>
</select>