1. Use the image code which other people stated.
2. Get the direct link of the image, you'll be able to know the direct link if it is showing the extension file (.jpg, .png, .gif) at the end of the URL.
Most people use imgur, but you can actually link any public URL(giphy, google images, etc.) as long as it is the direct URL of that image.
3. Use images below 2.5mb image size more than the will show an image proxy as the forum only allow that filesize. You can check it if you save the image, go to Properties>General>look for Size: <filesize>
1.5 does really suck, 2.5 would be perfect
OK, I increased it to 2.5.