You can do something like this, very simple and efficient solution: What i did was actually use a parameter instead of basic placeholder, created a SqlParameter object and used another existing execution method. For e.g in your scenario:
string sql = "INSERT INTO mssqltable (varbinarycolumn) VALUES (@img)";
SqlParameter param = new SqlParameter("img", arraytoinsert); //where img is your parameter name in the query
ExecuteStoreCommand(sql, param);
This should work like a charm, provided you have an open sql connection established.