Beside standard alpha channel in pngs, RGB(0,0,255) or in other words pure blue color
is rendered as transparency. There is one advantage of using it over alpha channel
- resized images look slightly better (or at least they did with the last version of FOnline
engine which I was using). The difference is large enough IMHO to stick with blue, at
least in the case of items' inventory pictures, which are usually resized to fit in the slots.
Another case where the difference is visible, is when player zooms in/out the map view,
so I would always use blue for transparency unless alpha channel was really required
to achieve some effect (like smoke, glass, soft shadows, etc) or it didn't matter at all
(like in the case of interface backgrounds).