I think you would find a little shadow around the rocks would set them into elevations. To made that is quite easy. Get a single rock onto a white background then contrast enhance so that it becomes an all black rock on a white background then blur that a little so it fades down. Then put the grey rock back on top. It will be a grey rock with a little dark halo around it for shadow. I believe you can do that on a transparent background instead and then you can use it as a brush. I'm not a Gimpy kinda guy tho