Do you know for sure if your image is saved with transparency?
I ask that as even though the image may have transparency the application or browser its displayed in / on may not support it.
Some forums may not support transparency so it matters not a jot which browser you use. They will also 'replace' any transparent pixels with a solid colour....in your example, black. White is the more common colour but it could be anything.
This of course does also depend on how you save your images.
Saving using "Save For Web..." is the preferred method...its what it is for....displaying images on the web with or without transparency.
GIF's only have on / off transparency. Any 'gradient' in it like a drop shadow for instance will have the background replaced with a solid colour....normally white. The drop shadow would still be there but over a white background not the page background.....this is why you sometimes see animated gifs with a nasty white 'fringe'.
PNG's are two-sided....PNG-8 acts much like GIF's...swapping the 'gradient' transparency for a solid colour.
PNG-24 though does the full transparancy without any problem.
JPEG's just dont do transparency, period.
So to ensure your image is good for all types of transparency use "Save For Web..." and PNG-24.
This wont of course mean it'll work on all forums in all browsers.....forums do convert uploaded images into a format they prefer regardless of you using PNG-24.....thus it removes all your hard work by re-placing the transparent pixels with a solid colour.....the one you used as an example obviously does this and uses 'black' as the replacement colour.
The above image is a PNG-24 image.....the right side is transparent and if you use the PSG-Dark theme you'll see the dark background....white if you any other theme.
The images on the left show the way different formats handle transparency.
Regards.
MrTom.