Eye Candy Illustrations is a a type of ranking tag. As the word implies, it is tagged for illustrations of eye candy.
For members who think of artworks that contain something which brings "eye candy" upon viewing will be attached with this tag. In a way, it is a form of ranking tag. Hence, there is no criteria nor there are any rules on what kind of artworks can be tagged. The artworks can range from moe, to delicious food illustrations. Hence any artworks that one think is beautiful enough can be tagged using this tag.
Since the definition is too loose and too general, there are various problems on the usage of this tag:
Since this tag is usually tagged subjectively by the viewers, what it is considered to be eye candy to that particular viewer who used this tag might not reflect the same way for other viewers. Even if questioned or doubted, it is hard to give a proper or stricter definition for it. There are cases where wiewers apt to use this tag below average illustrations, and fujoshi related contents, much less used even for guro related contents.
Since the whole usage on "Eye Candy Illustrations" is too broad and general, it has no definition, it can be used for any illustrations. Hence searching for pretty artworks using this particular terms will not help much.
While tags like 「10 Points Are Not Enough」, or 「Should Be Ranked More」 do contain similar problems stated above, 「Eye Candy Illustrations」 is considered to be the worst out of it, and also highly abused.
「Eye Candy Illustrations」 tag is usually given by users biased towards certain anime shows, hence it is very common to see this tag used for Copyright genre works.
And recently, Puella Magi Madoka Magica
Since they are all popular search terms (as well as the franchise in general) in pixiv, it is hard to find any non-popular shows with this tag.
Especially for K-On case, Searching for this tag will be accompanied with related tags like Houkago Tea Time, Yui Hirasawa, Tsumugi Kotobuki, and many more.
「Eye Candy Illustrations」 are used not only for artworks, but also manga works and font template works as well. It sometimes cause various questions and doubts such as "Why is this tag used?", "What's the purpose of using this tag?", "Is it cute?", "Is it a nice idea?", and "Is it cool?", none of which were made clear to the original artist most of the time.
Since the tag can also be used for all sorts of quality from delicated, detailed illustrations to sketch, users might think of it as a cheap praise, or even as an insult, instead of praising it.
Besides the problems stated above, involving the fact that this tag remains not properly defined, the usefulness of this tag remains very ambiguous up to this day. Since one can only tag as much as 10 tags, one should think up better tags to use instead of this.
Additionally, tagging each of particular artist's artwork with "Eye Candy Illustrations" will cause the tag list of the artist's page to take up unnecessary space, something that is considered unpleasant by many viewers.
Approximately 1000 illustrations will be picked and updated by the users on each article editing.
By the way, this is the oldest eye candy artwork submitted on pixiv. but wait...eye....candy?