1.  If you already have a SQL database, with a table that store an image with the SQL datatype "image"; currently the out-of-the-box SQL Service broker does not support this datatype, or any of the following below:

• 
  
• TimeStamp
  
• Image
  
• SQL_Variant
  
• Binary
  
• VarBinary
  
• Numeric
  

http://help.k2.com/onlinehelp/k2blackpearl/userguide/current/webframe.html#ws_sql_01.html

Perhaps a custom service broker can achieve this.

2.  Another thing that you can do is create a K2 Smartbox Smartobject, with a property of the K2 type "Image", a K2 type "image" will encode your image file and store it as an nvarchar(max) in the Smartbox database.