@Gkyr
I'm not good with this stuff, and I can never remember to type the brackets right. So, my way of doing it may be somewhat convoluted (esp to folks who are more comfortable with it), but it works for me.
I put the screenshot in Paint, crop, edit, add text, whatever and save it to my desktop. Then, I resize/move my browser window to get it on my desktop and still see the desktop file icons. Then, I just drag/drop the image from my desktop. Here's some pics of what I do:
When you click on 'Spoiler', a box pops up where you can put in a title; if you leave it blank and click 'continue', the title will be 'Spoiler' and the image above appears in your typing box. The cursor will be where the red arrow points up. Make sure the cursor stays there for the next step.
Then click the icon just to the left of the smiley face and this is what you get:
Then, I drag/drop the pic into that box. As long as the cursor stays between the two bracketed 'spoiler'(s) in the first pic, it shows up as a clickable spoiler. I'm still unsure of myself, so I click 'preview' first, then click 'Spoiler' in the preview and make sure it looks right before hitting 'post'.