The PNG format has an alpha channel per pixel that controls the transparency, so RGBA rather than RGB. I believe that setting the alpha to zero will make it transparent. There are no designated transparent colors so you cannot 'white out' parts of the picture to make them transparent. This means that although Strive will use PNG, JPG, and WebP image formats, most image packs will use PNG to make use of the transparency.
You don't need to remove the background if you don't want to, as some users have noted that since the Bugfix mod adds fading to the body images it is not as important as it once was. If you need help, the Discord (https://itch.io/t/284398/discord) has a channel where people are quite experienced at removing backgrounds.