One thing I would like to add about picture widths, is that you can use a percentage value. So if you want a picture to fill up the *width of a wall, forum or blog post, specify the width as 100%, not a pixel value.
If this does not work, then it's because the original width of the picture is smaller than the width of where you're posting it. You can rectify this by clicking on the image in the message editor to select it, then click the edit image icon and enter 100% in the first dimensions box in the edit image dialogue, the picture should then expand to fill the *width of where you are posting it.
*(Margins and padding usually comes into play to ensure that pictures are not squashed up against the edges of webpage areas because that looks a bit shoddy).