I think
SuperShouden should be able to give you instructions on how to do it.
I'm pretty old-school and untechnical myself. The only way I can think to do this is to open your spreadsheet and use the "Print Screen" function to capture an image of your screen to the clipboard on your PC. (It's usually Alt+Print Screen or Fn+Print Screen depending on your PC / Laptop type.)
Open Paint after doing this and do Ctrl+V to paste in the image as a bitmap.
Use the selection tool in paint to highlight the area you want to upload to

and use Ctrl+C to copy the selection. Open a new Paint window with a small initial bitmap size and paste in the selection from the clipboard.
You should now have a bitmap that looks like the selection from the excel spreadsheet that you wish to post. Now save the file as: give it a name, and save as .jpg or .png file type onto your computer.
When you post a reply at

underneath the reply box where you type your message you'll see the Image Upload option. Browse to the file you've just created, then click "host it!" to host the picture on imageshack.
Copy the direct link URL that it generates and paste in into your message surrounded by image tags (
)
There's probably far simpler ways to do this, as I'm sure many of our graphically adept younger members will soon point out for me!
*Edit* And have already done!
How'd I miss a tree that big?