I tried to save a PNG file using Paint 3D and it is not transparent. Don't know if it will even load a transparent file. You have three options. 1) Do it yourself using a free graphics program such as GIMP or Paint.net 2) Try an online converter. Don't know how good these are. 3) Post your image here in a new post and ask for assistance.
https://www.gimp.org/downloads/
https://www.getpaint.net/download.html
google search on convert jpg to png transparent replace jpg with your file type