Now a days, you find lots of Operating Systems packaged as a IMG file, meant to be copied to a portable disk like a USB pen drive. For instance, Ubuntu Netbook Remix and Moblin  are available as a IMG files. Now if you are interested in running these operating system flash images  in VirtualBox, you will have to convert the IMG file to a VDI file (VirtualBox image).

Here is how you do it.

1. Download the USB boot image (IMG file) of the OS and save it to a location on your hard drive.
2. Fire up a terminal and navigate to the directory in which you saved the IMG file.
3. Run the VBoxManage tool from the command line.

   $ VBoxManage convertdd  moblin.img  moblin.vdi

   Here, as an example,  I have converted Moblin image. You should substitute the name of  OS image file you downloaded in the place of moblin.img.

4. You may copy the newly created VDI (VirtualBox image) to a directory of your choice. Now it is ready for use in VirtualBox. (Learn how to create a virtual machine in VirtualBox).