I created a simple circle drawing in Inkscape. Then I exported the drawing as png.
When I resize the image in IrfanView and save it as png then a thin white border appears around the circle.
You can see the difference between both images here:
Why does saving with IrfanView put a white border around circle?
This is likely due to saving out a PNG-8 from IrfanView. PNG-8 has very basic transparency support, on or off. So with an anti-aliased edge like you have there, where you really need different levels of transparency, it is simply applying a transparency color map (taken from your white background color) and applying it to the pixels that aren't completely transparent.
To avoid this, make sure you are saving as a PNG-24.
