Well, Microsoft finally had an insight into the user complaints about not being able to download a (legal) Media Refresh ISO for people with preinstalled Windows 8(.1). Now, you just have to download the Media Creation Tool from http://windows.microsoft.com/en-us/windows-8/create-reset-refresh-media.
It’s portable, just execute the EXE and it’ll download you the selected Windows 8(.1) ISO you selected. Additionally, it finally accepts the Product Keys from Windows 8, which is a big step.