Here I took a bmp file that already existed without transparencies, sized it to 2400×3200 and used my utility to convert it to a PNG with black transparent. Black was already the background color.
A BMP will do some compression, but will never alter the original bitmap the way JPG does so this is a cean conversion.
If you have a photo you will have to outline subject and color the background in order to get a single color to make transparent. With RB new everything prints white you probably dont want white OR black transparent but some other color like a neutral gray.
You probably also dont want any print in black if it isnt surrounded by other non-transparent colors, if you want a black or dark TEE.
I am offering this utility free to RB members.