Generating an output PNG, JPEG or TIFF file for an image service in ArcGIS Open Data is essentially similar in function to the Export Image operation performed from the REST end point of the image service. Thus upon exporting, the image is generated at the default Dots per Inch (DPI) of 96. As the DPI of the output is low, the image is of lower quality when compared to the original service. This is known limitation of the Export function in ArcGIS Online, as the ArcGIS Open Data design is based upon the ArcGIS Online platform.
There are no settings available at the ArcGIS Open Data Admin page for controlling image download quality, as the raster format in general is not directly supported in ArcGIS Open Data. However, the following are some recommendations on how to improve the resolution of the exported image.
Increase the Maximum Image Size per Request parameter while initially publishing the image service. This allows users to increase the resolution when downloading at the default DPI setting. For more information, refer to ArcGIS Help: Parameters to control the image data.
Note: If working with an image service published from a raster dataset, this setting is changed at the service level rather than in the source data.
When working with a mosaic dataset as the source data, improving image resolution can be done by modifying the Cell Size in the dataset Properties. For the steps on how to do so, refer to ArcGIS Help: Designing a mosaic dataset for analysis.
Increasing the zoom of the image to a higher scale prior to downloading the image from the ArcGIS Open Data site increases the clarity of the image. The image is still exported in the default 96 DPI settings, but the quality of the image is less pixelated, and features on the image are more visible.