Change "Cannot read file picture.png !" into "Cannot read file picture.png ! - file doesnt exist" in case problem is that it didnt find the file.
I yesterday spent 1-2 hours bug trying to figure out what I had accidentally done for new GIMP saved PNG-files not to open, when older pictures in same GIMP session worked just fine.
I kept getting the "Cannot read file picture.png !"-error.
What the problem actually was, was that I had made a backup save of the main to a different folder, and forgot to resave/open back to the main folder.
Therefore, when I was adding new graphics to the right location, it looked like I was able to open old files (since they were in that backup location too), but not any of the new files.
Because of saying it cannot read the file, I thought point was that there was something unusual with the PNG-files instead of them not existing at all, and I tried saving them in different png-ways etc. and nothing worked, until I finally realised that I was actually using the main-file in the backup-folder.
Point here being, that if the message had been "Cannot read file picture.png ! - File doesn't exist", then I would have figured out the problem much faster. As just saying it can't read a file leaves too much interpretation.