You can also press CTRL+I to insert an image.
However, in Italics and Bold it mentions the same combination:
You can also select text and press CTRL+I or CTRL+B to toggle italics or bold respectively.
It doesn't make much sense to overload a single key combination with two distinct operations, and indeed it only works for toggling italics for me, not for inserting an image. And I believe that is how it is meant to work, because the correct combination for inserting an image is actually CTRL+G, which can be found in the tooltip appearing when you hover over the Image toolbar button:
I believe this should be reflected in the Help Center. Specifically, the above mentioned line from the Images section of the Markdown help page should actually read:
You can also press CTRL+G to insert an image.
replacing "CTRL+I" with "CTRL+G".