It took me a while to realize I could do this so I’m sharing the method.
When you drag an image from your computer into a new forum post window, you’ll see auto-generated text like this:
In the preview on the right side of the window, you’ll see how large the image is (you might have to click the x to hide the related posts).
To resize the image:
Add a comma and percentage after the resolution values (e.g. ,50%):
Be sure there are no trailing spaces after the % symbol or after the ] symbol.
You can also just type in new values for the resolution. So instead of 690x87, you could put 397x50. I like the keep the ratio the same so I usually calculate a new value by doing 690/87*50=397. This is sort of pointless since the % method above is quicker and easier.
Here are examples of a 50% and normal (100%) image: