Solve 404 error when background-image is not a valid URL (bootstrap)

- Added a `function validateURL(textval){...}` from http://stackoverflow.com/questions/833469/regular-expression-for-url

- Check if it's an image or if the background-image is a valid URL (Bootstrap use background-image to apply linear-gradient, for example: btn class, causing 404 ERROR)

- Leave the console.log's I used for better understanding.