not a computer expert but try this:
first, make sure your pic is not too big. there are free things that you can use to resize them.
when/if you hit "Reply" to a thread, look under the smilies (on the left is a big page of dancing bananas, drinking heads, happy birthday, this thread is worthless w/o pix, etc) to a line that has "File Attachments" a blank box, then a "Browse" button, and then an "Add this Attachment" button.
Hit the browse button and find the file name of the pic you want to attach, select the pic, OK then hit Add This Attachment. then move your cursor in the Reply panel where you want to add the pic, then below the file name will be in a location and you then "Add This" and after Uploading your reply, it should be there.
... or wait until Andy directs you to a thread that explains it...