Please follow the below steps to create a WES7 install image on a USB:
Step 1: Follow the steps(1-3) mentioned at the below location to create a bootable utility UFD
To copy the utility files to a UFD and make it bootable
From a Windows 7 or Windows PE environment, run the DiskPart tool. For example, at the command prompt, type the following:
The version of the DiskPart tool provided by both Windows 7 and Windows PE supports the partitioning and formatting of a UFD as a bootable device. Previous versions of the DiskPart tool do not support partitioning and formatting a UFD to be bootable.
Use the DiskPart tool to determine the disk number and device size to be used for the next step by typing the following at the DiskPart command prompt:
Use the DiskPart tool to partition and format the device and make it bootable. For example, if the UFD is Disk 1, type the following at the DiskPart command prompt, replacing <device_size> with the capacity of the UFD (in MB).
select disk <disk_number> clean create partition primary size=<device_size> select partition 1 active format fs=ntfs quick assign exit
The UFD is now bootable.
Step 2: Launch ICE (Image Configuration Editor) from the toolkit
Step 3: File -> Select Distribution Share
Step 4: Right Click and create a new answer file
Step 5: Add all the features that you want to include in your image into the answer file
Step 6: Goto Validate -> Validate Only (i.e. Validating the configuration/image) and resolve all the errors. Be sure to select "Bootable Windows USB Stack".
Step 7: Goto Tools -> Creat Media -> Create IBW Image from Answer File
Step 8: In the "Select the target folder for the image" specify the drive of the USB (eg. E: or F:), and then click 'OK'
Using the above steps you should be able to create the WES7 image on a USB. Let me know if you face any issues with these steps.