Background .png not showing in Chrome and Firefox
Solved!
It would appear that saving PNGs with Photoshop creates a PNG which isn't compatible with chrome or firefox. So instead, I opened the file with Fireworks, re-saved and the pngs are working.
I had the same problem with .png files saved from IrfanView: they would not be displayed as a background image before I unchecked the "Save Transparent Color" option in PNG save options. So I guess PNG should be saved without transparent colors to be displayed as background images.