Adding cards is done with the [ img ] tag. Put [ img ] at the start of an image and [ /img ] at the end (remove the spaces in the tag names).
Between the [ img ] and [ /img ] tags, put the URL of the image. If you are using imgur, that is the "direct link", not the "image link". The link should end with the image's file type.
An example of such a link would be:
https://i.imgur.com/6pnCyxt.pngThen you put [ img ] before the URL and [ /img ] after it (without spaces).
You can see this example under here by clicking "quote" on the top right of this comment. It'll show you the markup I did in order to post the image.
As you can see, this image is too big. I can fix that by adding "width=something" inside the first img tag.
[ img width=250 ] (IMAGE URL HERE) [ /img ] (without spaces between the words and the [ or ])
If you clicked "quote" you can see the complete markup under here: