Preview image for Sonic Mania model

[robowil]

I was able to get the textures applied on the Sonic Mania model for Sonic. I was also able to get him into a T-Pose. How do I make a preview image? I only have MS Paint, and I need to make the picture transparent. I attatched a picture of the Sonic model from Mania so you know how the model turned out.

   

But how do I make a transparent photo for the preview? This is my first time uploading a model from a game.