Welcome to Android Central! Open the email on your tablet, then tap the photo attachments and look for the Save or Download option. The photo files should appear in the /Downloads directory of the tablet's internal storage.
To transfer the file to a flash drive, the tablet has to support OTG (On-The-Go) adapters, and you either need a flash drive with its own OTG plug, or a separate OTG adapter to connect the drive to the tablet's microUSB port. You can then use any file manager app to copy the picture file from the tablet's /Download directory to the flash drive.