please see the Example below
http://www.aspsnippets.com/Articles/Display-images-from-SQL-Server-Database-in-ASP.Net-GridView-control.aspx[
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In your problem, you can change the SQL query to get blank images first.
select * From [YourTable]
ORDER BY CASE WHEN [ImagePathField] IS NULL THEN 0 ELSE 1 END, [ImagePathField]
Example for Small table.
select * From Emp1
ORDER BY CASE WHEN Name IS NULL THEN 0 ELSE 1 END, Name
Use This and first see the SQL result is correct. You need to come null values first in the output.
Please Marked as Answered, if you got a help.