Shouldn't be able to do this. Apps installed to an SD card are encrypted with an encryption key unique to the phone, to prevent the data from being accessible to any prying eyes (in contrast to the mess that was SD card support for Android prior to Kit-Kat, a security disaster). So shouldn't be able to access them, at least via normal means.
And why are you trying to get snaps? The point is for them to remain private, no? Seems like a big invasion of privacy in my opinion. Do as you please, but if you're going to save snaps, then at least have the honor to use the screenshot button (assuming you're using 6snap) and let the other party know what you're up to.