There is no such thing as a standard "best size". It completely depends on what the image you're posting, and for what purpose. Mostly though, as long as we could see what you're trying to show us in an image, then it should be good enough. Just don't post those unnecessarily huge ones that fills out monitor screens.
You can use this bbcode to adjust the image sizes without needing to edit them manually:
[img height=100]https://bitcoin.org/img/icons/opengraph.png[/img]
https://bitcoin.org/img/icons/opengraph.png[img height=50]https://bitcoin.org/img/icons/opengraph.png[/img]
https://bitcoin.org/img/icons/opengraph.pngJust change the height value.
The problem is when i upload a png file that shows a black background unlike your BTC logo, how to get rid of that? what type of file(png or jpeg) i do need for that
