We must first export it as a PDF, then change the PDF to a jpeg in Preview. Do do this.....
Open up the pages document. File / export - select PDF and export it out to your desktop
Once done, double click the PDF file. This should open in Preview.
Once open in Preview - file / duplicate - this will create a duplicate copy
Once the duplicate is open - file / save - in this window, change the format to .jpeg, and save it to your desktop
You can also do file > print, and then on the left corner there is a PDF button. Click on that, and then from there you can change the PDF into the JPEG. Also, many times, you can use a PDF instead of a JPEG.