If the employer is worried about how you represent their public image, and you've disassociated this activity well enough that the employer doesn't know about it, then their concern is mostly satisfied.
Anyway, let the mutual filtering continue. You don't want to work for a company that cares about drawing public dicks, and such a company won't want to hire you. It doesn't really even matter to decide whose attitude has a problem -- you're just culturally incompatible.
It's hardly even "acting unprofessionally". It's a lighthearted project where, well, some part of the fun is waiting for the first person to draw some dicks in. If you can't get over it, that's more likely problem with your attitude, not the person's who drew it.