I think this is because most have been rescaled. You'll find this happens anywhere online or on any computer when an image has been rescaled. They can't show images at their original size or it would overwhelm the system - and your internet connection. So one puts up with a little blurriness.
As for the ones that look fine - the original is probably a small size to start with so the rescaler has no work to do.
In answer to your original question - I don't think there's anything can be done to make them look better. Though you might check that there isn't extra blurriness at your end, by doing the usual things we recommend for making wordpress.com site and your blog run better:
Clear your browser's cache
Disenable all browser add ons, plugins and extensions then gradually reinable them to see if any have been causing the problem and if so which.
Update Flash (I personally think this is the most logical one to do if it's a problem with display of images, slideshows, or video)
Update your browser to its most current version, you can find all the most popular ones here: