With that error message it can be two things, the first thing is that the path is off so the error occurs when it tries the make the folder and can't because of the extra ../ in front of it. The problem could also be that you do not have the permissions to make this directory at this location. If this is the case you would need to make this folder and upload it to your server behavior you are able to upload to it.
To check to see if the problem is with the path you are using you can try updating it to be like this:
$WAGLOBAL_Images_Folder = "/images/image_folder/";
You should be able to get a positive result with this path as a value assuming that these folders exist or that you will create them. The slash in front of the first folder indicates the root of your site. So you should ensure that this value matches the structure of your site. If you still have problems with this let us know and include some values for the images directory in your site that you are trying to upload to.