How to subset an ECW format image and maintain its existing opacity?
In versions previous to ERDAS IMAGINE 2016, subsetting ECW images may cause them to lose the opacity channel. If using an older version you can try the Create ECW Transparency tool to add an opacity layer to an ECW image.
Manage Data tab > Image group > Create ECW Transparency
It is strongly recommended to upgrade the ERDAS IMAGINE software to the current version to take advantage of the latest improvements.