While she can theoretically sue you for defamation
for making false and damaging comments about her in the comments section of this photo, her chances of being successful on this cause of action are very very slim to none.
The elements of defamation are:
1) A false and defamatory statement concerning another;
2) The unprivileged publication of the statement to a third party
3) and Damage to the plaintiff.
First, she would need to prove that you were making a statement of fact that is untrue (that she is heartless). You can counter that it was your opinion that you were expressing. And, being heartless is really subjective, so it would be extremely hard for her to prove.
Second, she would need to prove economic damages as a result. This is very hard to do, and unless someone refused to work with her as a result of your comment on a photo (highly unlikely), she will not be able to do so.
She would be very hard pressed to find an attorney with any interest in taking this matter, so I think it unlikely that you will receive a summons. But, even if you do, you really don't have much to worry about, since she's lacking on 2 of the 3 component to a successful defamation cause of action.