A .ico is not just a renamed PNG. It is a small container that stores your icon at several sizes at once, so Windows can grab the 16 px version for a browser tab, the 48 px one for a desktop shortcut, and the 256 px one for Explorer's large view. Drop your PNG on the box above, tick the sizes you need, and this converter packs them all into one file. It runs in your browser, so nothing is uploaded and every size keeps its transparency.