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”."

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.

The OSM licence (the ODbL 1.0[1]) requires this of the attribution:

> You must include a notice associated with > the Produced Work reasonably calculated to make any Person that uses, > views, accesses, interacts with, or is otherwise exposed to the Produced > Work aware that Content was obtained from the Database, ... and that it > is available under this License.

i.e. the notice must be such that regular people using this site will know about this.

There must also be a similar notice on the produced map.

[1] https://opendatacommons.org/licenses/odbl/1.0/