Are higher-resolution images always better?

I have had this perception that for a given size, higher resolution pictures will always look better than the lower ones.

But on this link in StackOverFlow I read something different.

So my questions :

  1. For a fixed size will higher resolution images always look better ?
  2. Does this improvement, if at all, depend on the kind of display and underlying hardware etc (for example is this above case specific for Apple)
  3. Should I always try getting better resolution images? Or is there a limit after which human eyes will not really be able to differentiate this improvement because of resolution change ?

PS: I am not taking about the print qualities of these images, but the visual experience one gets from seeing them on screen.

I will clarify more.

I have to use an image of size 800x600px(Fixed).

I have 3 options : 900 dpi Image, 600 dpi image, 4oo dpi image, all in size 800x600px.

Is it always a good idea to use 900 dpi above 600 and 400 ?

And above stated questions follows with this data.

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