I asked a question on Stack Overflow, but the image I included doesn’t show up. Instead I only get an image icon. I used Gyazo to upload the image. Is that site not allowed?
Use the image uploader in the question itself. You don't link to an image, you instead link to a website where the image is present. You can see clearly because of the absence of a .jpg or .png extension on your link.
If you inspect the image in the link, and look for its source you will find:
Using the appropriate markdown you can make that show:
A quick way to get this link would be to right click the image available in your original Gyazo link and select "Copy image address".