Microsoft KB Archive/57955
INF: Windows 3.0 Icon and Cursor Compatibility with 2.1 ID Number: Q57955
SDKPaint automatically converts Windows version 2.1 bitmaps, cursors, and icons to Windows 3.0 format. These bitmaps, cursors, and icons are not downward compatible to Windows 2.1 after having been converted by SDKPaint.
After being converted into Windows 3.0 format, the icons, cursors, and bitmaps are then composed of device independent bitmaps (DIBs). DIBs are incompatible with Windows 2.1. Additionally, for icons and cursors, Windows 3.0 allows multiple DIB images within the same resource, which allows Windows to select the image most suited to the current device resolution.
Therefore, use Windows 2.1 SDK tools to create code and resources for applications to run under both Windows 2.1 and Windows 3.0. Applications built with Windows 3.0 tools will only run under Windows 3.0 (and later).