Let me try to write up my view of the "Naked" tag in one spot. I've said repeatedly that it is the single most complicated tag in the game. I will also note that there are some distinctly different viewpoints among the packmakers and outspoken game players as to exactly what level of nudity would qualify for the tag. So the following is just my opinion on the matter.
First I will note that one of the things that complicates the tag is that it works in three different ways given what other tags it is with. The three are 1) Normal use, 2)use with a sex act tag, and 3) use with a farm tag.
Naked is a modifier tag. It is meaningless by itself. Instead it is used to modify when the pic will appear given the other tags on the pic.
I'll start with the last, since the first is the most complicated.
3) For Farm use, Naked is effectively a sex act of it's own. The scary elf lady actively trains in enduring nakedness. (Group and Bisexual are also sex act tags when in farm settings) In this specific case it is not a modifier tag. "Beast Naked.jpg" is perfectly valid. Beast says it's a bestiality farm pic, and Naked says to use it in that setting for Naked training.
2) Outside of the farm, if any of the four core sex act tags are used, the game automatically acts as if the Naked tag is present, whether or not it actually is. End result for this is that "Naked Sex.jpg" and "Sex.jpg" will function identically in the game. The Naked tag is pointless and redundant in combination with any of the four sex act tags (Sex, Anal, Service, and Fetish)
1) Ignoring for the moment the issue of what level of nakedness qualifies for the Naked tag. For it's use, with some exceptions it means that the pic should be used when the game expects the girl to appear naked. This comes up in Nakedness training, but also once you reach the point of being able to tell the girl to stay naked all day or all the time. (Or she's a Naturist.)
I tend to, in my mind, work like there is a scale. Normal < Libido < Naked. Normal is regular pics. No tag for that. Libido is for pics that are getting a bit sexy, but she's not at the point of actually being naked. Finally Naked is for pics where she is showing herself. I'll note that the game makes a big deal about the effort it takes to get her to the point of being willing to show skin. Thus it's also a big deal to me to not have pics without the Naked tag when what is shown should not be happening in the context of the game. If she is waitressing while topless, that should be Naked, or the pic will get used at other times.
So, finally the issue of what qualifies as enough nakedness. And I'll say again, there are definitely some divided views on this issue. Ultimately, as a pack maker, you need to decide where you draw the line yourself.
For me, I tend to go by what was in the BK Name tool's tool-tips for a long time. If the image is showing tits, pussy, or asshole of the girl, it qualifies as Naked. Note that if tits are showing, but covered with thin cloth, I'll often tag Libido instead of Naked. Remember that Naked is about the state of mind of the girl. In that she's being sexy, but still is technically covered, even if nothing is left to the imagination.