varbinary is the SQL Server datatype used to hold variable-length binary data
SQL Server's varbinary datatype is declared with a length specification in the CREATE TABLE statement:
CREATE TABLE tablename (..., colName varbinary(n), ...)
where n is the maximum size of the column in bytes (limited to a maximum of 8000), or the word MAX if more than 8000 bytes might need to be stored. (The absolute maximum size is approximately 2gb as of this writing; consult SQL Server documentation for the exact current maximum.)
For more information, see this entry in the SQL Server online documentation.