I tried seeing it was any better in Firefox by zooming. I also tried to use Preview to do this and it just got really blurry.
The basic problem is that you're looking at an image (now you could use photoshop if you were to have it by chance), but even easier is to create a photobucket account (it's free and great for putting all images/photos onto DU) to view and edit images. Drag the image (which is really easy to since you seem to be using a Mac like me) onto your desktop, upload the image to your album, and then choose to edit the image by moving your mouse over it and selecting edit. Photobucket has two editing modes, one is a real options and the other is complex, then either increase the size of the image or put into full screen.
You can also do this basic process with iPhoto, which you probably have as part of your normal Mac applications package. By using iPhoto to open the jpg and then increasing size and/or viewing in fullscreen.
http://photobucket.com/ is great for uploading images to the internet, and it's how I and I'd imagine most of the posters here put images on DU because it recodes the image to a unique link so you're not violating copyright.
Good luck! PM me if you have trouble or further questions.