Just open the Window File explorer. you can drag and drop image file from the explore into MD. see my post on http://www.marvelousdesigner.com/forum/show-off-and-learn/10/tutorial-video-quick-start
Unfortunately that doesn't work. here's my own video:
You can insert texture images into MD by changing the setting of Window vista or 7.
1. Enter "Control Panel"
2. Click "User Accounts and Family Safety".
3. Click "User Accounts".
4. And click "Change User Account Control settings"
5. Down the scroll bar to bottom, "Never notify".
6. And then, restart the computer.
7. Then open a folder with texture images, drag and drop into MD. :)