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 that you can't get the facility of persistence storage with the Live USB, which means every boot to the Live System will be new.

Using dd

First take note of the device number of your USB drive with the command sudo blkid. Most probably, it will be /dev/sdb or /dev/sdc. Almost all of the time, it cannot be /dev/sda.

Then open a terminal with the Ctrl+Alt+T shortcut and use this command (I'm assuming your USB drive is /dev/sdb).

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.

Using cat

You can also use the cat command. Open a terminal and enter the command like below:

sudo cat /path-to-ubuntu-iso/iso-filename.iso > /dev/sdb; sync

Replace the /dev/sdb with your system-specific USB drive number.

Please note that you need to specify the output device as /dev/sdb or /dev/sdc, not /dev/sdb1, /dev/sdc1, etc.

Update for the updated question

Not all distros 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 distros.