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: http://img1.3png.com/340af8a9dd2e666738f797b6573a963e1e8f.png. 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.