If you press the "-" (minus) button while reviewing images, you can view thumbnails instead of single images. Press a couple times and you'll see smaller, but more thumbnails. Then you can scroll quite quickly to the last image. Press the "+" button to get back to single image review.
Just to be sure we're all on the same page, when you say "last image", do you mean the previous image or the final image? In other words, in your example where you're reviewing image #50 of 100 - do you want to go to #49 or to #100?
I have the same frustration. There is no direct way to do it that I can find in the manual. You can use the multi-selector centre button to toggle full-frame and thumbnail views and then once in thumbnail, the command dial to quickly rotate forward or back. See pages 219, 309 and 318 in your manual.
You can also use the zoom (-) button to display 4, 9 or 72 thumbnails. When showing 72 it takes the camera some time to load them up but then you can move very quickly to the end. Quicker to turn it off and back on again
When I turn of my camera and then power back on and do an image review, it DOES NOT reset the display to the last photo TAKEN, but to the last image viewed. So if I where viewing #50 of 100, it will still return me to 50, even after power reset.
The only way back is to take a dummy image, view that, delete that...
I don't mean to hijack this, but a related question on navigation.
If you have two cards, say in backup mode or whatever mode, and you want to switch cards, the only way I can find is to hit "-" until they are really tiny (which takes a really long time), and then you can scroll to select the card.
Is there a faster way to select the card?
And a slight variation on the OP's question -- say you are at 100 and want to go to 50. Is there a faster way back to it? Scrolling is slow, hitting "-" to go back is also very slow.
I've found myself several times with a shot of a specific play, and later in the game (often more than once) someone says "can I see that again". It's a real pain to get back to them quickly, even if I know which number.