Are you positive that you are compressing and pre-sizing images offline to the exact dimensions required by your theme, prior to uploading them as featured images? If you aren't then that will be the cause of your problem. What's critical in the case of featured images is their dimensions. There are specific dimension for featured images in each theme and if you do not pre-size the images to the correct dimensions prior to uploading them there will be issues.
With Suburbia, you can enlarge and feature your most important posts by marking them as Sticky posts. http://en.support.wordpress.com/posts/post-visibility/#sticky-posts
Suburbia theme description > http://theme.wordpress.com/themes/suburbia/
The custom header image is 155 by 155 (width, height).
The featured image works best with images at least 350 by 248 (width, height).
live demo > http://suburbiademo.wordpress.com/