If you are uploading your own image, then you will need to "post reply" to existing message.
You will get a big white space to type into.
Above the white space are things like "B" for bold, "i" for italic etc. There's also an "Img". Clicking this puts these into your text:
[img]..[/img]
You can then insert the http:// link to the image, between these two bracketed [img] statements.
BBCode is explained on the hyperlink to the right of the white space
Uploading your own image
Most file types are supported.
If you are uploading your own image, then you will need to "post reply" to existing message.
You will get a big white space to type into.
a little below that white space are three tabs (options, upload attachment, poll creation).
select "upload attachment"
You can then hit the "browse" button to select the image on your computer.
Then hit "add the file"
Once it's uploaded, you'll have the option to "place inline" which will allow you to embed it among your text.