How to make first post image as feature image above <head> tag


I want to post first blogger post image as feature image in above < head > tag, so it will display on top of the blog. i use following code but it not working can you help me out, code is.

< img expr:alt=’data:post.title’ expr:src=’data:post.firstImageUrl’ expr:title=’data:post.title’ />

to get img url i use above code and not working.

please can you tell me the proper code to display it on above < head > tag. and i also want to set its width to 100% my site is here using Square Magazine blogger template.