Some image files you downloaded from the internet might be in the incorrect format. You can open it in PhotoShop, GIMP, Paint.NET, … and re-save it as *.png or any format you want. This problem occurs sometime if you are using Chrome, but it doesn’t occur if you use Firefox.

For example: You can try to save this image: Although it says it’s a PNG file, but it’s actually a JPEG file. If you don’t believe it, you can open it in Notepad, Notepad++, … You’ll see the JFIF at the beginning lines, if it’s a PNG file, you should see NG.