That just needs some
anti aliassing. You can also try to compress it a tad less. It's now 76 KB, 100 KB is the forum limit. Look
here for some great tips.
To enable anti aliassing, right-click on your desktop and click properties - settings (tab on the right) - advanced and then click on the tab of your videocard. If you don't have that option, you should get the latest drivers for your card. One other possibility is that your GPU (videocard) doesn't support anti aliassing. If it doesn't, it's best to turn on anti aliassing in the game. (don't turn it on in the game if you're videocard does support it!)
When taking shots, it's always a good idea to set all the sliders (in the game AND your videocard settings) to the highest quality and run the game in the highest possible resolution. That way, the game might turn into a slideshow, but at least the graphics are optimized and you don't need high framerates to take a screenshot.
After you have taken the shot, size it down to 800 pixels wide (max). That will also remove some jaggies.