[kazkada]

Generated PNG does not mention OpenStreetMap. It seems that it violates OpenStreetMap requirements laid in http://www.openstreetmap.org/copyright/en:

"How to credit OpenStreetMap

We require that you use the credit “© OpenStreetMap contributors”."

[rmc]

There's also slightly more requirements than that. The credit must be placed so a reasonable user will see it. This is important if you have (say) an app. You cannot just bury the credit deep in the settings screen.