Is it possible? I only found the NSFW one
Perhaps like this?
I see. It’s a good way to hide a spoiler, but the image that goes into the post’s URL field can’t be hidden?
No, the whole point of an image post is to display the image.
If you want to make a post with a main image, but in a spoiler; make a regular text post (ie ignore the url and just use the body) with the spoiler at the top.
Yup.
::: \spoiler Perhaps like this?  :::
(just remove the backslashes)
There is also a button for “view source” on the website (between downvoted and reply).
Lots of people miss that option and end up asking how something was formatted. I’ve taken to just displaying it when demonstrating to avoid the extra questions.
