Glad to help before, Ben.
What you've experienced here is just another intrusive thought. If you looked at that picture and up popped that horrible thought/image out of nowhere then that's a classic intrusive thought. It found a possible trigger and went with it.
Like much of our subconscious thinking these thoughts can be triggered like any other panic/anxiety scenario. It's no different to "see lump, thought pops up about cancer x, y z, etc".
The thing is, they are finding the mind just works like this. Other subconscious thoughts just pop up and there are subtle triggers. Think of it a bit like seeing a chocolate bar and thinking you want one because they have always been ones you liked or a smell bringing a memory with feelings with it.
It's just an overactive subconscious looking for stuff to do and an overly sensitive conscious reacting too strongly in a negative manner to it.