A raster image is rectangular and has a set number of samples and lines. If your data do not fill all of the pixel cells, then there is always a background of some value. You can change it through masking but there still a value there. In ArcGIS, they have the concept of a 'no data' value where these pixels take on this special value of 'no data' and appear transparent when displayed but the pixels cells are still present. If they want the background to be transparent, then you can set the Background value to ignore in the image header and when displayed (in ENVI 5) will be transparent.