It's an interesting in-game feature, many of the actual existing nebulae can be visited in Elite Dangerous. Unfortunately not every nebula has its identical picture.
However rather many nebulae are indeed real-based imitations, some of them more detailed than others.
Size, color, structure, luminosity, density and stellar content are what makes a certain nebula unique.
The origin of birth of the nebula determines most of the visual aspects. They are therefore categorised in several types such as dark, supernova remnant, (proto)planetary, emission and reflection nebulae. They can even be futher categorised by special properties, or most often their shape like spiral, disk, ring, helical, bipolar and so on.
As of the many nebulae that can be discovered, there are actually quite a lot out there which are so far away that they can never be reached. FSD-tech would need to double the parameters if commanders ever wanted visit the currently out-of-reach nebulae. Even then, with a neutron star FSD-boost or FSD-injection enabling a jump range over 400 Ly it would be too short to get to the kings and queens of the nebula heaven. Or perhaps it's not the range that compromises travel, the ring nebula is an example of one that is within range of our FSD-technologies but can't be jumped to because there's no star in or near the nebula to go to.
But then still, the ring nebula can be seen from a distance. Those that can't be taken a picture of are the ones to call nebula kings and queens.
Yes, the Lemon Slice nebula is one of those. Commanders have to be tortured by the great expectations of it's beauty, which we can't take a picture of because it is...
... beyond our reach.
- CMDR Diedericque