In the top right of the reply box you will see a little speech bubble saying "spoil". Click that and then enter the title of the spoiler. For example, if you are selling a robot head, entitle the spoiler, "Robot head". Then insert the image inside the brackets that you have just created. Here is an example of what it should look like (Remove the *):
[SPO*ILER="Exam*ple"]ENTER YOUR IMAGE HERE[/SPOILE*R]
To enter an image, right click an image from a website and select, "Copy image URL". Then go back to your spoiler tag and press ctrl + v to paste the URL inbetween both brackets ][.
For example (remove *)
[SPO*ILER="Exam*ple"]
![](Something*something*something.j*pg)
[/SPOILE*R]
You must put [*IMG] and [/IM*G] when using an image.
Hope this helps.