Im making a blogging script. I want to be able to switch between HTML and just typing text. I need to know how to do sumthing like that I can type sumthing liek this:
Hi
This was the thing
lalalal
and it will automatically put in line breaks, but I want to be able to put in images like this:
<img src="lalala.gif">
and I need to know how do I make a large text box like one in this message board when posting a message. How would I go about doing all this. Or is there a pre-made script I can do all this in?

What you want to use is a sort of BBCode function to eliminate any funny business and attacks. A sample BBCode function for using an image would be like:

[ img ]http://www.domain.com/image.gif [ /img ]

Using reg-ex, you could replace the above sample to the proper html img tag. As far as the textarea size, using css width and height to define your size. If you want the same functionality as WT, search for wysiwyg javascript textarea.