I don’t think most straight guys would be quite so sanguine if they started receiving dick pics on their phone.
They'd probably just secure their phone, shrug their shoulders and go on with their day.
Derec, in addition to what I said just now in my post above, I can agree with you that sometimes, the term sexual harassment can be overapplied, and possibly in a small number of cases cited for the wrong, mistaken, irrational reasons or in a very small proportion of cases malicious reasons, but not in this general case. Yes, it may arguably not be as serious as other forms of harassment, especially if the sender does not repeatedly send pics to the same person, but at the same time I think it is still valid to call it sexual harassment, by and large, or at least potentially alarming nuisance, partly because of the prevalence and the imo valid fear that the person sending it might possibly have more serious ill intent (which the receiver can't know).
If you want to call it a relatively mild form of that compared to others (such as overhearing a dirty joke for instance, which in many cases clearly wouldn't even be harassment imo even if it might in some, depending) that might be better, imo, because seeing a mystery dick pic is more tangible and aimed at you personally than for instance hearing something said which is not necessarily about you, and it carries a potential risk that something more serious could occur, even if it's not physical assault (some form of stalking, following or pestering for instance). We have seen how some women can attract this even just by walking down the street.
I'm not suggesting it should be a crime, but if it happened in, for example, a workplace, and was sent by a co-worker, I think it should definitely be reportable to those in charge. It should be reportable in any instance, actually, but not necessarily treated as a crime, imo, unless it was part of a pattern of harassment towards a particular person by another or others or crossed a reasonable or legal threshold in some way.