I do direct from photobucket but have done directly from my computer.
Click on the paperclip ( attachment) and it's to the right of the smiley icon.
That will open a pop up window where you browse and pick what photo you want...click 'open" then click 'upload'. It will tell you to wait,...when finished, close pop-up..go back to paperclip and attach.
( I did a 'sample' to be sure I was telling you the correct steps LOL )