well.. I hope you have your pic on the computer already?
the allowed file size for jpgs seems to be 195kb (strange..), so you need to get your picture below that.
There is a nice small application that may help you:
http://www.bluescreen.de/imgresize/us/download.php
try to get it below 160kb, then write your comment with the reply button.
If you scroll down, there is the "Manage Attachment" Button.
click it, choose the photo, do the upload and "submit reply"
that's it
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edit: I should read...
do you have it upright on your computer?
If yes, save again and under save option make sure you disable "keep original exif" options or something like that..