It's one of those things that are very hard to write a precise rule about. Basically the smaller the differences within a set of wallpapers the less versions we'll accept.
Usually when someone uploads several different versions of the same wallpaper (or a different version of an existing wallpaper) it's up to staff members' best judgement to decide whether or not they differ “enough” to keep the multiple versions. There are many ways in which wallpapers can differ, some of them more acceptable than others.
Size is the most important one and this is actually where we have a fairly simple rule of thumb: We keep one or two default resolutions (e.g. 1920x1080 and another in a different ratio) plus always the highest resolution available.
Next we have various types of color management. People will change background colors, change the wallpapers overall hue, saturation, contrast etc. Usually we reduce those down to a small set. For example yesterday I reduced a set of over 10 slightly different versions to 3 by merging the other ones. Obviously this process is heavily influenced by personal taste, though I try to keep the ones that already have the most favorites.
Like I said, the amount of different wallpapers we'll accept in this case usually depends on how different they are. For example if someone uploads the same wallpaper in color and monochrome, that's usually okay. But if they follow up with another 5 versions in between we're likely going to remove a few.
(edit: I just removed a bunch of @Amixtra's that you mentioned)
Next we have those wallpapers that aren't just colorized but actually have some heavy filters applied that change the picture itself. More often than not these fall into the category of “low quality edits”, because they are usually done by beginners. Again it depends on the individual case whether or not it'll get deleted.
Lastly there are those wallpapers where a user has added some small detail to the image, often some text or logo. If it's additional detail within the image itsel (like in the example in the post above) we'll usually keep those. However once again if there are a lot of them and they all only differ in very minor details we're going to reduce them.
However one thing we definitely don't accept are people who pick a wallpaper and add their own name / pseudonym / website etc. We're not an advertisement platform for youtube channels, or blogs or whatever. Whenever there are versions with and without some random dude's misguided watermark we'll pick the one without. Feel free to report those as duplicates.
PS: In case someone is wondering why we don't just keep them all as long as there's any difference at all (“the more the better”): Our goal is to offer a wide variety of high quality wallpapers. People who scroll down our latest page or search results want to see something fresh, not pages on pages of the same handful of wallpapers. Sometimes it is justified to have a small set of different versions because everybody will actually prefer a different one. But at a certain point it just gets annoying to everybody and the site's overall usability will suffer. We don't want that. Scrolling through our wallpapers listings should be fun, not tiresome.
