Argh! Now you've said it I can't un-see it!
1. Write your text and add your pictures in the text editor.
2. Drag the mouse (i.e. holding the left button) over the text and/or pics you want to hide
3. The area to hide is now highlighted blue, at which point move to the icons at the top of the text editor, and you will see a black coloured speech bubble centre left - same row as the "add a picture" and smiley face icons etc. Click that speech bubble once
4. A pop-up window will appear with the text "Warning: Spoiler" in a text box at the top, marked "hint", and your highlighted stuff in a text box at the bottom. Change the hint/title from "Warning: Spoiler" to anything you want e.g. "Here are my testicles - look!"
5. Click "Submit" on the pop-up, and then submit your post as normal using the blue button at the bottom. The content you highlighted should be hidden behind a text line that says whatever you wrote for "spoiler tag" title.like this (Click to show)
[Insert genitals here]