Step 1: have a regular PNG image with a transparent background
Step 2: use TinyPNG or similar services to compress the image. Using image viewers and editors on your computer will show that the image is essentially identical in looks.
Step 3: upload the image somewhere
Step 4: use the image link and place the sign: you will notice that the transparent background turns black, and the rest of the image has a strange effect similar to dithering that heavily lowers its quality

Note: this dithering like effect that ruins the image applies to non-transparent images too.