Recently, I took more time to generate merely Branding icons for my Microsoft Teams App. Finally, I got a very good site which converts a png image to transparent
Steps to make a png image as transparent:-
1. Open above site
2. Upload your image to "png" section
3. In the "png transparency creator option" section, make "similar color tones" to 70% or 100% as you like so.
4. In "transparent png" section on right site, click "save as" button and download your file
5. Once done, please upload your downloaded image to Microsoft Teams Branding Section
6. Download your package file.
7. Try to validate it in the Validation section of Microsoft teams.
Tell me your comments and let me know how it goes for you.