Displaying 500px wide image in blog that is clickable to show full size image

Just moved to a new server and upgraded to the newest version of wordpress. Now having trouble to find the same functionality as before with putting an image in the post.

Uploading a large image took up the whole screen. so added this to the theme functions.php file: $GLOBALS[‘content_width’] = 500; made the image only 500px wide.

this enabled me to choose the image display size when uploading images, but the images are not expandable when you click as they were with the previous version before upgrading. what do i need to do?

thanks in advance