An Amazon Fire HD 8 Tablet is 7.6in by 4.5in which is not a standard picture frame size. The two closest sizes are 6x8 and 8x10; and 8x10 was the smallest canvas frame I could find. In either case, it is necessary to mat the tablet's screen to make it look good in the frame.
Unfortunately, the tablet's screen size is 6.8x4.2 inches which doesn't match standard mats. I had to have one cut at a local framing store. Custom mats turn out to be a bit pricy; it cost $20 which is a significant cost for a bit of cardboard. I could have made one by hand by cutting a hole in a piece of card, but it looks better with one done professionally with the appropriate tools. Alternatively, if you're less impatient than I, you can buy these for $2 at various places on the Internet.