Use the image button and copy & paste the image link (link must end in an image extension, like .JPG). Then select the image (so it turns blue), click on the SP button, and then click ok. Basically same procedure for video, just use the "media" button instead of the image button.
Or manually add
"[s-p-o-i-l-e-r-]" behind the image, and
"[/s-p-o-i-l-e-r]" after the image. (But without the - and " )
Same with media, just replace "spoiler" with "media".