The decision about artwork is not made by iTunes/Apple, it will be the company that owns the recording (the record company) or perhaps the band who made the music.
Talich wrote:
What I mean by that is that it's not at all necessary that the inline cover has to be exactly 600x600?
No, it is not necessary for artwork to be exactly 600x600. If I look for my own artwork, I might use 300x300 on one album and 600x600 on a different album, it simply depends on what I find online.
There is no one hundred percent standard in the resolution of embedded covers? When buying music from itunes store.
No, there is no standard, and the decision about artwork for music sold in the iTunes Store is not made by Apple, it is made by the record company that owns the rights to the music.
Record companies do not adhere to a standard. They can be inconsistent about the genre of the music, lazy about the text that they type in and as for the artwork, it appears it can be inconsistent even on one album.
From my own experience, I think every three-CD album that I own has one disc with different information to the other two. A typical example is that discs one and two might be titled "Album Title", while disc three could be "Album title Disc 3". What this will do is split the 3-CD set apart, with discs one and two as one album and disc three as a separate album, even when the tags state disc x of 3.
If you want higher resolution artwork, copy and paste it from wherever you can get it. Remember though, larger resolution pictures take up more storage space (even on your computer) and very large resolution pictures will be scaled down to a lower resolution when added to an iPhone etc.