Alright, so lemme see here.
- Poor Quality Image, used to mask any Photoshopping.
(but it could be that the user zoomed in to see that image from a larger pic or had a terrible camera)
- The source is a dubious website who sourced from 4Chan from a post in FB or something which no longer exists.
(a source has hinted it might be true, and 4Chan has correctly leaked information before, too...and if the FB post was deleted, it could be that the original source has realized what NDA he/she may have just broken or had been ninja'd)
- Generic Logo Font does not really fit with Mario
(but it does fit in part with Mario Paint's)
- The Hand is reused (but it has been reused). It is cutoff at the right moment and is supposedly pixel-perfect to a Google Image of the same hand. (if this can be shown, that might be the biggest argument against this image)
- Odd to have this setup on Thursday and not have it covered up, either
- Also odd that nearly half the banner is just ground (though it could be argued that the point is to advertise Mario Maker to those far away rather than those already in the booth).
Now, here's what works for this image.
+ Lighting and floor fixtures etc. all match the E3 location. We have not sourced this photo to any other E3 booth building photo.
+ The Smash Brothers logo and characters are lined up and shown realistically on the banners. If faked, the faker would have had to not only make the Mario Maker banner but also correctly orient the Smash images, too. Were those images present at E3 2013? (although as mentioned above, the banner does not really match those other three, which is odd, but not implausible...)
+ The idea of Mario Maker is not absurd like other fakes that are around. Also, leaks like these are not new to Nintendo (see NSMBWii a la 2009).
Honestly, unless we have proof of the hand being pixel perfect or find a source image for this photo, it is increasingly likely that this is real. And if it is real, cool. A bona fide Mario level editor would be pretty cool, and from the banner it looks like things are not just traditional Mario elements, too (stacked Goombas, winged SMB Goombas, Hammer Bro leaving pipe, etc). Are coins usually placed like that on bricks? Just remember that this is still Nintendo...it might only stick with original Super Mario Brothers tilesets.
My only concerns are whether this is not the last leak (I hope nothing else leaks for their sake) or if this was one of their major announcements (still a few left for sure I hope). I suspect Wii U eShop. If this is an NFC game, there must be a real good reason for it. And as we see, that is a Wii U stylus; unless the game is cross-platform, Mario Maker is likely a Wii U game and NOT the unannounced 3DS title.