1) Zoom/scroll until what you want to appear in the jpeg is in your display window.
2) Select File -> Plot... The following dialog will appear (minus all the pretty colours).
3) In the red area set the Printer/Plotter to PublishToWeb JPG
4) In the green area set the paper size to the desired size. VGA 640X480 is a good trade off between decent resolution and actually fitting in browser windows without side scrolling
5) In the blue area select Window in from the drop down and then click on the Window< button. This will return you to the drawing. Draw a crossing window around what you want to appear in the jpeg.
6) This step is optional but in the yellow areas I like to Center the plot and set the scale to Fit to paper. Also make any changes to Plot Styles, Viewport options or anything else you'd like.
7) click the Preview button. If everything looks good go ahead and plot it. You will get a dialog asking for a file name and location; name it something that makes sense and put it somewhere you can find it.
8) Upload and link into forum messages as usual.