From version 10.1, Mobile Development features the Asset Editor. Here, you can define one asset file. The asset defines how an icon looks like, and Mobile Development creates different sizes based from this asset definition. When migrating a mobile project that was built with a version prior to 10.1, you can define asset files as described in Defining Resources for a Mobile Project. Then, locate your old icons in the your_project/gen/.assets/graphics directory and delete them.