Cannot Center Page Content

The Feature Images on our blog were not displaying properly (they were being automatically displayed at 495×150 in the blog page and 100×150 in page). I implemented the instructions I was provided in this thread: https://kriesi.at/support/topic/blog-post-feature-image.

These instructions worked, and the images are displaying as one would desire, but another problem has arisen. The main text content on all of our pages is now displaying off-center. Everything has been shifted about 100px to the left. After various attempts I have been unable to correct this problem, and am unsure about how to proceed. I obviously do not want to remove the image code, because then the images will display improperly again, but at the same time having the entire site off-center looks odd. Ideas about solutions?

Thanks.