I’ve seen versions of this icon/sign both with and without text. (With the latter, it ranges from the unadorned “No Dog Poop” to somewhat more witty/chastising messages.) I think the icon/sign works just as well without text, if for a slightly different reason. It may take the viewer an extra second or two to gather the meaning from the message, but, once that meaning is gathered, I think it may have even greater potential to make an impression.
The only “skills” required by the viewer are fairly fundamental observation skills–having observed (and…”absorbed”) a dog defecating at some point in one’s life. Such an image, seeing a dog in such a stance is difficult to misconstrue or–in all likelihood, once perceived–to dismiss easily.
Once the information is transmitted, the emotions elicited are likely to be some combination of mild aversion and/or shame–primarily on the part of the dog’s caregiver, though such signs may also have the effect of turning parties not directly involved in the exchange (i.e. neither the property owner nor the dog’s caregiver) into interested parties. In the former case, as such a striking, direct image is not exactly a genteel way of getting the point across (e.g. using only text and saying “Please curb your dog”), the intended audience might be slightly shocked into compliance. In the latter case, an initially uninterested party may find the sign so affecting that, the next time they see another person not scooping their dog’s poop, they may–the image flashing into their mind–suddenly perform something like a citizen’s arrest–e.g. “Hey, aren’t you gonna clean up after your dog?!”