What is HEIC?
HEIC is what an iPhone saves photos as, and it is not really an image format so much as a container holding HEVC video frames - the same codec used for film, applied to one still picture. It stores a photo at roughly half the size of the equivalent JPEG, which is why Apple switched to it, and it is also why so little else opens it: HEVC is patent-licensed, so most browsers and a good deal of desktop software cannot read a HEIC at all. Large photos are usually stored cut into tiles that have to be reassembled, not as one picture.
What is PNG?
PNG is the web's lossless image format: compression that never degrades the picture, and a real alpha channel for transparency. It is the right choice for screenshots, logos, line art and anything with sharp edges or text.