ArturBonoVox, welcome to TG (I don't think anyone has said that).

Please don't feel overwhelmed by the response - you've picked a scenario that has a number of possible solutions.

I'd go with Gary's solution and for best results use an image twice as big (in pixels) as the size of the lightbox and turn on the Retina option.

As you can see this is a friendly and very knowldegable group.

Xara takes the complication out of web design (well mostly).