Posting from our CMS

I have a custom written CMS and I want to use it as we update our site to create blog post. The post will have one photo, title, and post content. I’m fine with writing my own stuff to ftp the image over and post directly to the database, except I don’t understand the data that is put into the postmeta table for the _wp_attached_file key. A recent manual post through the system inserted the following

a:5:{s:5:”width”;i:300;s:6:”height”;i:200;s:14:”hwstring_small”;s:23:”height=’85’ width=’128′”;s:4:”file”;s:74:“/usr/local/psa/home/vhosts/4realblogger.com/httpdocs/wp-content/joy264.jpg”;s:5:”thumb”;s:20:“joy264.thumbnail.jpg”;}

Where does the s:74 and the s:20 come from? As I see those values changing for each post, but can not figure out where those values are being set.