ok,
1 on the quick reply at the bottom page, you will see a blue button called 'go advanced' click on it.
2 another screen will load find the font and sizes bars, along that row, you will see a paperclip icon. click it.
3 a window will open in the top right corner there is a green button called add files. click it.
4 a little window will appear, at the middle bottom of it it will say select files, click it.
5 find the psd file and click open. (bottom right hand corner of window)
6 you will then see the file in the window in step 4.
7 now at the bottom of that window, you will see 3 buttons. click 'upload files'
8 the file will upload and appear in attachments bar in the window in step 3
9 in the bottom right corner of that bar, click insert inline and then click done
10 click 'submit reply' at the bottom of the message you are sending.