I have been passionately wondering why graphics that u design(not pix) using png(source code as *dhtml would say) still lose quality when zoomed. I used to zoom without any loss of quality but now the story has changed.

An image will lose quality if you zoom it beyond the size at which it was designed.
In simple singlishinesse:
1. If you design an image at say 500 X 500 px
2. And you zoom it to maybe 1000X1000 px it will loose quality.

Quality will start getting lost once you get to more than the original image size.
That is assumin that you have exported properly in loseless png format