How do you put a picture on an answer/post?

There's a icon with a picture of a mountain on the toolbar.
Put your mouse over it, it says insert image.

Two ways:
- you can use the Insert Image icon in the toolbar and insert the url to a picture on the web
- if the picture resides on your computer you must use the "Go Advanced" button instead of the Quick Answer method, then upload the pic using the Manage Attachments button.

Thanks a lot

It keeps saying that the file is too big, how can I make it smaller?

You'll have to resize it. I use a free program called XnView, or you can do it on the web at Wiredness.