How to change image url?

Go into your WordPress Dashboard > Settings > Media and then untick the option that says

Organize my folders into Month and Year based folders

This will make it look like:

http://example.com/wp-content/uploads/imagename.jpg

Then to take it further you need to update you config.php file and add in either this line:

define( 'UPLOADS', 'image/'.'files' ); 

This will give you:

http://www.example.com/image/imagename.jpg

Or this:

define( 'UPLOADS', ''.'files' );

…to get:

http://www.example.com/imagename.jpg