Additional field for each item being ordered in mp_order

How and where can I edit the code to have just 1 more field added into a product being ordered? Sorta like a special instructions for each product ordered, instead of Special Instruction per order like how MarketPress.

I notice in post_content for post_type "mp_order" the following content.

a:2:{i:2855;a:0:{}i:2858;a:1:{i:1;a:6:{s:3:"SKU";s:10:"TSHIRT006M";s:4:"name";s:34:"Yellow Long Sleeve T-Shirt: MEDIUM";s:3:"url";s:67:"";s:5:"price";s:5:"24.00";s:8:"quantity";i:1;s:8:"download";b:0;}}}a:10:{s:5:"email";s:23:"";s:4:"name";s:7:"aaa bbb";s:8:"address1";s:8:"123 ROAD";s:8:"address2";s:0:"";s:4:"city";s:7:"ANAHEIM";s:5:"state";s:2:"CA";s:3:"zip";s:5:"92801";s:7:"country";s:2:"US";s:5:"phone";s:10:"6262226666";s:20:"special_instructions";s:0:"";}

I need to insert 1 field, say after quantity, the content of this field will be populated with URL of an image.

I hope you get what I mean. Truly hope some one can give me some hints on where should I be looking at, please be specific.

Thanks so much!