Force the use of %random_image[%post_title%]% (by google images) | CyberSEO Pro | Support Forum

First of all, you can simply add it into your post template as described, with examples, here: Login to see this link

Also yes, you may select some non-existing content source like media attachment or the "thumb" custom field if they are not defined in your settings, so the alternative image source will be used. But I suggest you to use templates. That's what they are intended for.