I have an image with alpha channel, I need to save it as 32-bit BMP. However, Microsoft Paint doesn't allow saving as 32-bit, only 24-bit. I have Photoshop CS5, but it also doesn't have the option to save as 32-bit BMP.
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:
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: