Click reply.
Below the text box, on the left hand side, there is a link that says click here to upload.
A box will appear that says "Select a file to upload:"
Click Browse.
Select the file of the picture that you want to upload.
Its title will appear in the box next to the browse button that you selected.
Select "OK."
It will tell you if you successfully uploaded it.
If so, click "OK."
If not, click "Cancel" and go back and shrink the image.