Lubuntu USB - initramfs) unable to find a medium containing a live file system

0. Please tell us about your computer (it makes it easier to help)

- Brand name and model
- CPU
- RAM (size)
- graphics chip/card
- wifi chip/card

- Which version of Windows are your ruing?
- Which version of Ubuntu/Lubuntu are you trying (14.04.4 LTS or another version)?
- Are you ruing in UEFI mode or the old BIOS mode?

1. Please check that the iso file was downloaded correctly. Check with md5sum according to this link: UbuntuHashes

2. Try to flash the iso file to the pendrive with another tool. I think cloning is the most reliable method. In linux you can use mkusb, but it seems you are starting from Windows (with yumi). And you can use Win32 Disk Imager to clone from Windows. See the following link

Win32DiskImager/iso2usb

Good luck