black background on jpeg I need it to be white

I have for a few years taken photos with my iPhone airdropped them to the mackbook.

Then used the tool erase background it then turns the picture file to png, and I ending saving it to jpeg to have the file kinda small. and the background goes from transparent to white. well that's have not been a problem.

But now since last upgrade I do the same thing but the background now goes BLACK and that don't work I need it to be white.