As I mentioned before, I am not currently at my computer so I am going strictly from memory.  On the right side of that window there is an icon which controls the resolution of your image.  Basically it controls quality and sizing.  IIRC opening a separate window from that icon will grant you a selection of transfer rate in kb.  It is here that you can instruct the software to reduce the image to be transmitted @ 512 kb.  Once set, you can save your image for posting.  

Again, I can provide screencaps later if needed.