When exporting a map as a JPEG, BMP, or TIFF file format with the Background Color field in the Export Map dialog box specified to No Color, the output map yields a solid color background, such as gray, instead of transparent.
This is a known limit. The JPEG, BMP, and TIFF file formats do not support the No Color pixels. As a result, when the map is exported, the No Color pixels are assigned to a default background color value set in the Windows display settings.
The following workaround shows how to to obtain a transparent background when exported from ArcMap:
The following output is the an example of an ArcMap document exported as a PNG file format with the White - White combination. Exporting in a PNG or GIF file format with any same color combination for both the Background Color and Transparent Color fields (except for the No Color - No Color combination) yield the same output. The EMF file format also yields the same output. The exported map below is compared against a green-colored background to portray the transparent quality of the exported map.
Warning: For the PNG and GIF file formats, setting the No Color - No Color combination for both the Background Color and Transparent Color fields yield an exported map with a solid color background, such as gray. The same output is obtained if users export the map in the JPEG, BMP, and TIFF file formats with the Background Color field set to No Color.