To scale your images down, add a "width=x" in your initial img tag. The height will scale down according to the image proportions.
For example, if I had a huge image that I wanted people to see at 300px wide, I'd use this: [img width=300]
I haven't tried the other variants on these forums, but I think they should work.
There's this variant: [img width=x height=y]
As well as this one:[img={width}x{height}]