I need to use the LPC313xImgCreator.exe utility to create image to burn on LPC3141 board via DFU.
Ware can I download this tool?
Miki
Download the latest LPC313x CDL from this site or lpclinux.com. In lpc3xxx_cdl\csps\lpc313x\tools\ you'll find UnsImgCr.exe. This is to create an unsecure (no AES encryption) image whether booting via DFU or otherwise. I didn't need an encrypted image, so I don't recall what you need to make an encrypted image.
Also keep in mind that the LPC3141 is discontinued as per NXP's website. If you need AES decryption, you should go with LPC3143. Otherwise, go with LPC3130 or LPC3131.