You will be fine!

Getting tattoos are illegal in Iran but in recent years, tattooing has become increasingly popular among youths even full body tattoos.

For the foreigners travelling in Iran with tattoos, of course there is no problem except that you might absorb attentions. And your case with a porno tattoo on your arm which is not visible to be attentive, so how can it cause hassle for you. Read about Iran Dress Code

Easiest way to get Iran Visa