Update: See stackoverflow.com - Mount Android emulator images

Short Answer is, there are 3 types:

• Regular filesystem images, that can be mounted with -o loop option
• YAFFS2 filesystem images, which currently cannot be mounted in Ubuntu and have to be unpacked
• EXT4 filesystem images that have been further processed by the Android toolchain and need to be converted to regular images with simg2img from the Android toolchain.

So, you have a binary image extracted from an Android device, is that correct?

What does the file command say about the file? Something like this?

$ file system.img 
system.img: VMS Alpha executable

That system image is extracted via nandroid from a Samsung Galaxy i7500. Most of those old Android devices used yaffs as a file system. Knowing what you are dealing with greatly helps. (i.e. Which device are we talking about? Where is that image from?)

Converting images into other file systems will only produce garbage at best if you do not have installed support for the file system in question. Unfortunately there is no package for Ubuntu. But that question has already been asked and answered: Create yaffs2 file system on Ubuntu Server 11.10

There is also the possibility that this image is encrypted. But why would someone give you an encrypted image?

Since this is mostly about Android and you call yourself an Android Developer, please read the Android documentation, again. It's supposed to be all in there. If not ask your question at android.stackexchange.com, XDA-Developers, RootzWiki and the like. And please do not put a ROM on the net for hundreds of people to use unless you REALLY know what you are doing and you read the documentation. Doing it for fame is dangerous and bad for the users!

Select a virtual machine by clicking its name in the VirtualBox window
Click the Machine menu at the top of the VirtualBox window, and click Settings
Click the Storage category in the Settings window
Right-click in the storage tree pane, and click Add Floppy Controller
Right-click the Floppy Controller device, and click Add Floppy Device
Click the Choose Disk button in the prompt window that appears

Navigate to the floppy disk image file (.IMG) on your computer and double-click it
If that doesn't work, try renaming the .IMG as .ISO and mount it.

If that too doesn't work, use VBoxManage's convertfromraw command as follows:

VBoxManage convertfromraw --format VDI [filename].img [filename].vdi

Finally, mount the VDI as a hard disk.