Ubuntu Netbook Remix is available only in img. Its designed for installation on a netbook.
An img file is meant to be installed to a USB flash drive. A 2GB SanDisk Cruzer Drive is available from Wal-mart for under $10. When you've downloaded the Ubuntu Network Remix img file, the next step is to download Win32Disk (its free) from Source Forge, extract the contents to their own folder.
After that, insert the USB flash drive into a USB port. Windows will recognize and install the device drivers. Now run WinDisk graphical interface, select the drop down box to the flash drive and locate the file to be written to the flash drive. Select Write. You may get a warning it could corrupt the drive. Ignore it and wait until Windows finishes writing the img file to the flash drive. When finished, click exit on the WinDisk interface to close the program and remove the drive.
Insert the flash drive into the Dell netbook's USB port. Go into the bios to make the netbook boot from the flash drive. Either run the Live CD or select install. Its all very simple. A lot more Linux distros will be written in img rather than iso in the future simply because it can be installed fast to a flash drive.