You can use the famous (and also dangerous) Linux tool
dd for this job. But take note that, it will completely erase all the contents of USB drive. Also note, You can't get the facility of persistence storage with the Live USB, which means every boot to the Live System will be new.
First take note of the device number of your USB drive with the command
sudo blkid. Most probably it will be
/dev/sdc. Almost all of the time, it can not be
Then Open a terminal with Ctrl+Alt+T shortcut and use this command. I'm assuming your USB drive is
sudo dd if=/path-to-the-iso-file/filename.iso of=/dev/sdb
The command will run for some time. When you see the prompt
$ in your terminal, you're done.
You can also use the
cat command. Open a terminal use a command like below:
sudo cat /path-to-ubuntu-iso/iso-filename.iso > /dev/sdb; sync
/dev/sdb with your system specific USB drive number.
Please note that, You need to specify the output device as
Update for the updated question
Not all distro can be made bootable using the dd method. One such example is OpenSUSE DVD (not the Live One) . So, it is not possible to use dd to make a bootable iso for every Linux distro. But the dd method should work for all Live disto.