You are right. Using the photo/s, belonging to someone else, does not, automatically, make a person a scammer. Such person may, or may not, be one.
However, the negative effects, of using another person's photo (without owner's consent), could, at the very least end, upset the photo's owner &/or could be viewed (by others) as unethical or suspicious (esp. if "offender" does not own the depicted animal). Even if the "offender" did so without any malice intended &/or without realizing they were doing wrong.
At most, it is copyright infringement (and owner, of photo, is displeased).
Sadly, there ~are~ too many scammers out there. So, another negative effect is that, whether such person (using the photo of another) is a scammer, or not, it does have the possibility of bringing the thought, to someone's mind, that such person could, potentially, be a scammer (if it is not obvious, of course).