When mosaicking several images and reprojecting to a different map projection the output mosaic image has areas of black pixels around each of the individual input images. How can these black background areas be removed from the mosaic output?
The black areas surrounding each input image is an inherent problem due to the fact that a raster image has to be rectangular. The nature of raster data is such that when imagery does not fit squarely within an raster file, NoData (null or background) values are inserted around the image to form a rectangle. This happens during the reprojection of individual images. Just because the actual image area has a non-rectangular shape, it does not mean that you will have a file that contains no data outside of the image area. Instead there are background pixel values in those areas to form a rectangular image.
The solution is to select the Compute Active Area option when adding your images to MosaicPro and set the Boundary Search Type to Edge. Selecting these options will compute a polygon around the edge of the imagery ensuring that the surrounding background area is not included in the mosaic process.