To be honest, after eight years, I'd lay good money that she already knows you're masturbating but I'm sure there are lots of people who do the same thing, both male and female. If you don't come clean then you've got to be prepared for being caught out one day and then dealing with the consequences of that.
I think as you get older it's hard to match your sexual desires exactly so it's natural that there will be times when probably both of you want to masturbate when the other is not around or not in the mood. People masturbate for all sorts of reasons - they love having a good orgasm, they love following through when turned on or for other reasons such as the release of chemicals after a hard ay which then makes sleep a bit easier ... The fact that you choose to masturbate to images of women that don't look like your wife doesn't in my mind necessarily raise alarms - I guess we all wonder at times how it might be with someone different, be it older, younger, a different hair colour, different physique or even different gender - and we all have fantasies that can involve these types. To me the issue would be why you masturbate to other body types - perhaps because somehow that means the sex with your wife is different and thus the two things are separate? Maybe you need to explore that and work out why?
I've had several relationships with women who were very attractive yet didn't have a good body image so good for you for making your wife aware of how attractive you find her. Too often we get an image in the media of what an attractive woman looks like and in my experience it's nothing like what appeals to me - personally I prefer the natural look, I don't go too much on boobs and piercings and tattoos - but we're all different, so do keep telling her how attractive she is.
In the end, I think this may well come down to an awkward conversation sooner or later but in my opinion if you 'fess up' before she finds out then it'll be better. You may find that you both want to masturbate (solo or together) more as a result - with one person I was with we even had a code word which meant we were off to do just that [when we could see that the other was not up for it].
Sorry if I've rambled and good luck!