If you access Nearmap in a GIS package via WMS (Web Map Service) or Tile API (Tile Map Service), you can usually choose to download the imagery via either the JPEG or PNG image format.

We strongly recommend using JPEG as it is a more effective compression format; images should download more quickly and use less data. It comes at a very slight cost to image quality but the difference should not be noticeable.

In ArcGIS, for example, you can control this on the Layer Properties page.