Well the most common way is to change the file from .PNG to .JPEG. It reduces the kb greatly, It will also reduce quality, but not by much, Let me show you o:
This is my current signature, and a PNG file, Check the KB out, it's almost max,however, look at the image below.
Theres barely any quality change in the two, and, check the kb out, way less than a .PNG file
:3 Hope that helped
Edit: However. JPEG files will not handle transparency, so, If you plan to do a pop out image, I suggest making the backing of it first, saving as a JPEG, opening back up, and putting the pop out image on it,and then saving it to PNG that's how I do it anyways, way less kb, :3.