Your avatar is about the only pic you can post directly from your computer (at this point anyway). I took a pic I liked and sized it down to the correct dimensions (100x100 pixels I think), and then saved it on my desktop. In the avatar area you can browse for the pic you want. Just make sure to hit the "update" button next to the "browse" button, then go to the bottom of the profile page and click "save".
To upload photos, they must be hosted somewhere online (your website, flickr, etc). I believe Flickr is easier, but the way I have to post pics from my Mac gallery is to right-click the image, select "copy image location", and paste it in the pop up window of the post (click the picture of the tree in the bar when you're posting). You can then size the image however you want. I find about 4-500 pixels wide is about right to get the whole image in the post.