Application Express :: To Automatically Update The Value Every Time A New Row Is Inserted

I have a procedure I need to feed a number into in order to generate some thumbnails.

I'm trying to automatically update the value every time a new row is inserted but I'm having trouble with the code.

DECLARE

varInt NUMBER :='SELECT MAX(IMAGE_ID) FROM IMAGES';

begin
create_blob_thumbnail(varInt);
end;I get the error +'ORA-06502: PL/SQL: numeric or value error: character to number conversion error'+

If I run the query SELECT MAX(IMAGE_ID) FROM IMAGES, it returns the last row ID I created just as it should. The create_blob_thumbnail(varInt) procedure works fine if I manually put in a number.