@Dominions Son
But a lot of photographers who do a lot of image editing actually prefer to work with their camera's raw format (uncompressed).
Originals and files not meant for transmission should never be compressed with a lossy algorithm. But since everything is a compromise, you never know all the conditions that led to the choice of compression. Disk space may be at a premium or ignorance of the user could be the cause.
PNG is weaker compression, but it's lossless.
Not always. 24bit PNG (8bits per colour) supports lossy compression too. But the default is lossless compression and lossless is the only option for indexed colour PNG (8bit).
With lossy compression, every time you edit and re-save the image, the editing software has to re-compress the image, and this means that every time you edit and save, more data is lost*.
True. That's why no originals should be saved in a lossy format. Lossy compression should be applied only to final images that are meant to be viewed and not manipulated. But again, compromises may be necessary.