If its one of your own pictures you'll need to upload it to somewhere like Photobucket first. If your just linking a picture elsewhere on another website, right click on it and click Properties - that should allow you grab its address.
2. To show the image in your post, put
after it without any spaces between them and the address (or enable the advance editing options in your User CP for easy clickableness!)
I don't think there's a minimum requirement for piccies, I've never heard it mentioned before anyway.
Like Hammock_man says, get signed up on Photobucket (free), upload you piccies from your PC to your album on PB, right click on the IMG code under your required piccy then right click and paste into your bcuk thread.