in the horizontal window change the 100 to 56
in the vertical window change the 100 to 56
click ok go to file and then Save as - see fig 9
fig. 9
click to enlarge.
Make sure you have filetype JPEG selected and click save.
entering 56 gives you an image size of 448 x 336 pixels and suitable for adding to your posts as an image attachment or for using on an image host.
On imageshack this is what a 448 x 336 image looks like when added properly. You can still get a good image showing what you want for others to see and the forums page does not scroll sideways so the image meets the forum rules for image posting.