PAINT how to force save to PNG?

Question

Image processing

Can you please tell me how to make PAINT save the result of my work only in PNG? Such a strange question arose due to the fact that at work an engineer draws diagrams only in PAINT (and in the version for XP, although it costs 7), because it is so convenient for them. But because of the .bmp format, the files weigh an unrealistically large amount ("stung" the folder from 1.58 GB to 16 MB by simply converting them to PNG).
PS
PNG is required to maintain transparency on the diagrams (such a feature of the work).

Answer 1

Script for conversion. And run it in a folder on a schedule.
You can use XnView with its nConvert, you can use some ImageMagick.

Answer 2

GNU Dia /thread
And to be specific, your engineers probably don't have that name for nothing, because the wrong use of the wrong tool is very far from an engineering approach. And yes, the default format in mspaint is immutable.

Answer 3

It is worth saving once to png, as Paint then suggests png by default. (I have Win7)