Do you have it on a web site somewhere? if so click the yellow box in the toolbar above - the one that says "Insert image" when you hover over it - and enter the URL of the picture.

if you don't - I suggest using a site like ""PicOodle! - Image Sharing"" - just use the bit on the right "Upload an image" - find the image on your PC. Upload it and copy the URL (the one called "Direct link to image URL of image") it gives you into the Yellow box above.

Hey presto.