I'm not sure what it could be. Maybe something with the heading picture? I'm not really sure, but that's the only thing different I can see that you have that I don't in any of my first posts.....Could be totally wrong though
Just as a REALLY random question, which I would imagine you have, but just in case...have you tried removing and then just re-selecting the section you want spoilered and re-adding it?
And have you tried just doing part of the selection, like one line at a time to see if there is a point in which it breaks? Maybe seem silly, but might point to if there is maybe something specific. I don't think it's the image, since I just tested on my own posts and was able to add images into spoilers on the first post, but I can't get it to do what you are seeing so just a thought.
It works for me, and I can post images in spoilers in the first post of a thread. I have checked the settings for permissions, and there is nothing there about normal members posting spoilers or images in the first post.
Ok, in digging into it a TINY bit more, if I inspect the code for the page on one of Mojo's posts vs one of mine (as mine also works Cor) the spoiler tag is NOT getting parsed, so it's not creating the code. When I look at mine it has all the code built around the spoiler with the oncick event handling for showing/hiding the text, etc. So there is SOMETHING that is happening. I still think starting with removing/re-adding and checking if it works line by line might provide more insights as to what might be causing it. Just my thought