It would all depend on your particular phone. If you have a way of getting it from your phone to your PC then you can do it just like any other picture on your PC. Create a new post or reply and on that screen you'll see in the "Additional Options" box a button to "Manage attachments" - click on that and it should be pretty self explanatory from there. Note - if you are doing a quick reply then click on the "go advanced" button just below the quick reply box to get the full reply page where you'll see the manage attachments button.