Access Data that is stored in Primary FileGroup On SQL Server through DBLinks on Oracle
Filegroup is completely irrelevant: a table doesn't know where it is and internal storage allocation or placement won't affect a query.

If you have a single MDF you most likely only have one PRIMARY (or default) filegroup anyway

image datatype is deprecated too: use varbinary(max). 

Have you run SQL Profiler to see what Oracle is sending to SQL Server? Is there a call actually being made?

Edit:  why does the 2nd trace use a table called "P44730584C4264287A49ABEAF_ADJ"