As you note, candid can refer to being honest. (There are a few other meanings that aren't relevant except etymologically, going back to the first meaning, which is "white").
As such, it refers to not putting on a front or persona. Hence when you pose for a photograph, it is not candid, but when someone just takes a picture of you, it is candid.
Now, that is a pretty well-known meaning of the word candid in the context of photography, but to understand the results of your image search, we need to consider it combined with what is considered sexually attractive and the economics of information.
It probably won't be much surprise to you if I point out that a large number of people like seeing women wearing very little. For that reason, photographs of women naked, in bikinis, positioned so that you can see the silhouette of their legs, positioned so that you can see their knickers (or lack thereof), engaged in sexual acts or acts that the audience fetishise, and so on, are valued more than other photographs, by many people (primarily, but not exclusively, straight and bisexual men).
Now, of the group of people who like seeing women wearing very little, there is a subgroup who will expend energy on actually seeing them, and a further subgroup who will expend energy in producing and publishing them.
And of that group, there are some who favour unposed photographs, ranging from some who find a natural smile more appealing than a posed one through to a sneakily taken photograph that takes advantage of a telephoto or hidden lens (the panchira shot beloved of the paparazzi) or even breaking and entering to take revealing photographs of women who do not consent to them.
All of those latter are (or claim to be, some may be actually posed), matching a definition of the word "candid", though they range from a position that avoids anything pornographic through to a level only engaged in by criminals and Daily Mail journalists.
Further, within the community of people sharing such photographs, it is candid that is the differentiator; the context assumes that everyone is talking about photographs of women (or of men, but the demographics involved make them less common as a whole) and so candid is what differentiates one set of photographs from another. Hence, while in general, candid has little to do with most of such shots over any other unposed photograph, there are large swathes of the internet in which that is the very particular meaning it has in context.
Despite deliberate skewing against (so as to make search results more diverse across the range of results that would suit different people using the same term), such pictures are hence valued by the network as a whole, and so just as good money chases out bad according to Gresham's law, so pictures of women wearing relatively little will have a natural dominance within the results for any given term. The network understands these pictures as "good", and favours them.
So, it's not that candid means much different to how you define it in your question, it's just that the particular cases of candid that come up in terms of what photographs people choose to publish on the Internet has a heavier leaning towards two-piece swimming suits than one might otherwise expect.