Use the post thumbnail for the single blog pages too. You can use the add_image_size() function to create an additional image size that is displayed on the blog posts.
add_image_size('single-post-image', 650, 250, true); // name, width, height, crop mode
Then in single.php
Simply place the first function in your functions.php, and the second in single.php. You can then wrap it with whatever you want to control the formatting. This way your formatting will always be perfect and you don't have to worry about sizing images exactly: they're all done for you.