Well, I won't post anything like that again. I won't even post the Captain Rainbow stuff just to be on the safe side.
Don't feel bad for that, I'm sorry if it offended you.
You made it with innocent intentions, the problem is that Internet is a bad place, and since it's a bad place, there are also people who do bad things.
In fact, there are some people who do fan-arts similar to that one, but with different intentions than yours.
This may not be the case, though it can be misinterpreted as one of those cases by other people who aren't aware of your intentions, sadly (this also includes outside people who accidentally watch the monitor while the picture is on the screen).
That's why it's better to keep it in a spoiler; there is nothing wrong with the picture itself but it's better to leave people the option to hide it.
This doesn't mean that you should stop posting artworks, by no way. As long as there's nothing that can be misinterpreted or make people feel uncomfortable, then it's ok to post stuff.