Use the excel zoom function to make your diagram fit nicely on your screen
press "print screen" key
open Paint (accessories-->Paint)
paste the screenshot in there: edit-->paste. Press escape.
use the select tool (dotted line rectangle) to select the area of your picture
Edit-->Cut; File-->New; Edit-->Paste
now File-->Save-as to a JPG or PNG format
You should be able to upload that.
Out of curiosity, why excel of all things, for drawing a picture?