Changing the cover picture
I finally got impatient and clicked to correct it again on June 2nd. Still no fix.
FINALLY on June 5th the cover image was the correct one I tried to change it to on May 25th. https://www.bookcrossing.com/---/15522096
I don't know anything about the inner workings of computer programs, so I can't imagine why it takes so long, but it did finally show the correct image, so don't give up hope.
what are jpg and png?
Image formats. They are file formats that are especially for images. Most pictures in your computers are saved as .jpg files, and .png is another option.
(This is why I despise the default settings in certain operating systems, that do not let the user of the computer see the extensions; if you cannot see the .jpg or .doc or .png at the end of the name of a file, you have no idea what you are dealing with and it's that much harder to know what's going on.)
Any image editing program should be able to open a .jpg and then save the image as a .png. I don't know what you are working with, and so I can't be more precise than that.
Some old ladies are programmers, by the way. There is nothing inherent about old ladies that has anything to do with how good they are with computers. So I'm not sure why you're bringing that up. Sorry, dear Kirjakko, this happens to be one of my pet peeves!