Most small children don't need telling: they tell me.
I'm at the other end of the age spectrum and I need everything thread-to-needle, and then some. I hate to comment on other people's instructions, especially when they know more than I do, but I know from experience that adding pictures on forums can be complicated till you know how.
Resizing can be an issue. If the file size is OK when you try to add it you will get a green tick on the right hand side. If you get that tick you have managed it and then it's just a matter of where you want your picture to appear in your post. If you click "Preview" it will show you where it intends putting it. If you want it in a particular position, put the cursor where you want it and click "Place inline" then have another preview to check.
If you get a red "X" rather than a tick, you need to re-size. I find MS Paint the easiest way to do it. (Paint is still there in Windows 10) Open your image in Paint, then choose the resize option. I can never remember the maximum permitted size so I just reduce the pixels by trial and error.
I've been there before and got very frustrated so if anything needs more clarification - so long as it's simple enough for an old simpleton - please ask.