Easiest way [to post your desktop], not using any 3rd party software...
1) Take a snapshot of your desktop. I find it easier to just hit cmd-shift-3 (although some like to use Grab for this)
2) You now have a file on your Desktop, typically called Picture 1.pdf, open that in Preview (should be the default when double-clicked, but Acrobat/Acrobat Reader etc. may have messed with that)
3) In Preview choose File -> Export... (cmd-shift-E), you now get a dialog box, choose Format: JPEG, and hit the Options-button
4) You get a new dialog called JPEG Options, choose Best Depth, slide the Quality Slider to Best, check the Target size button and write 100000 (some prefer 102400), and press OK
5) Back in the Picture 1.pdf-save dialog you navigate to an apropiate place where you want your Desktop picture (in JPEG format) to be saved (cmd-D will take you directly to the Desktop in all save and open dialogs) and give it a name (if Picture 1.jpg isn't good enough)
6) You now have a JPEG of your Desktop under 100kb on your Desktop (or wherever), this can be attached to a MacRumors post, using the Manage Attachements-button under Additional Options, when you Reply to Topic (or Edit Post). Click that button.
7) You get a pop-up where you can Manage your Attachements, under File to Upload click the Browse-button, find and OK your Picture 1.jpg in the dialog box, and hit the Upload-button. A message will appear: "Uploading File(s) - Please Wait", and after some seconds (more if you're on dial-up) your file will appear under "Current Attachements". Now you can "Close this Window"
8) Finish your post and hit the Submit Reply-button and you have posted your Desktop.