By using left or right whitespaces you can choose the alignment

{{ wiki:php-powered.png}}  For left align
{{ wiki:php-powered.png }} For center align
{{wiki:php-powered.png }} For right align

Of course, you can add a title (displayed as a tooltip by most browsers), too.

{{ wiki:php-powered.png |This is the caption}}

If you specify a filename (external or internal) that is not an image (gif,jpeg,png), then it will be displayed as a link instead.