Graffiti is plural, but it has comfortably evolved into a mass noun and may be used with singular verbs. In fact, since the 1980s, the singular mass noun has far exceeded the technically-correct plural. Both the graffiti are and the graffiti is can be considered correct with one exception: if you use graffito in your writing, pair graffiti with plural verbs.
Though rarer, the singular of graffiti is still graffito and should only be used with singular verbs. Avoid graffitos, which only serves to remind readers that the writer struggles with standard and correct plural forms.