1. Open your doll in Irfanview, then click File -> Save as.
Make sure PNG is still selected as the file type. Check "Save Transparent Color" in the save options box, then click Save.
2. Select transparent colour
You will see a zoomed-in picture of your doll. Click the background to mark it as the transparent colour.
NOTE: Before making your doll transparent, you should make sure that the doll itself does not contain any of the background colour, otherwise that will become transparent too. The background in MS Paint is pure white by default, so eyes and white clothes may cause problems.
3. And there you have it:
Now it's time to upload your doll to the internet.