To post a theme image you need to do the following:
1) Make sure you have used the theme image and it is a finished theme image
2) Create a new topic with the name of the theme as the subject
3) Upload the theme image to an image hosting site such as ImageShack
4) Paste the image link using the Img button above
5) Test the download by right-clicking on the image, saving the picture as a BMP and opening it in Xpadder