One way I've used is have a TV for your image link say named "ImageLink"
Create your snippet:
if( $TVarray = $modx->getTemplateVarOutput(array(ImageLink)) ){ $tvOutput = $TVarray[ImageLink];
if(empty($tvOutput)) return '';
else return ''.$tvOutput.''; };
Assign the the TV to your template, add the tag [*ImageLink*] wherever you want the image to appear.
Add the call to your new snippet in the template.
If you add a path to an image in the "ImageLink" TV it should appear, if the TV is empty then nothing will.
EDIT - changed to a snippet