How to save image as 32-bit BMP?

Solution 1:

Photoshop >> Menu >> File >> Save As >> Choose format BMP in dropdown box >> Save button >> Choose 32-bit format >> OK button

However, photoshop will save the image as XRGB. In case the image has alpha channel (as ARGB), must use another tool:
http://www.qualibyte.com/pixelformer/

In Pixelformer, choose Menu >> File >> Export, Choose export file type as .bmp, Click 'Save', the following BMP options dialog should be shown, choose 32bit ARGB:

Solution 2:

Paint.Net supports 32-bit ARGB bitmaps via a plugin available in the Paint.Net forum.

To install the plugin:

• exit Paint.Net
• download the zip
• extract the dll
• copy the extracted dll to C:\Program Files\Paint.NET\FileTypes

As noted in the forum post, you need to rename files to *.bmpx before opening them in Paint.Net to avoid conflict with the default BMP filetype handler.