SQL Server 2019

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CREATE TABLE mytable(
   Id     INTEGER  NOT NULL PRIMARY KEY 
  ,UserId INTEGER  NOT NULL
  ,Image  VARCHAR(16) NOT NULL
  ,Day    INTEGER  NOT NULL
);
INSERT INTO mytable(Id,UserId,Image,Day) VALUES (1,3,'3_20200408_1.png',1);
INSERT INTO mytable(Id,UserId,Image,Day) VALUES (2,3,'3_20200408_2.png',1);
INSERT INTO mytable(Id,UserId,Image,Day) VALUES (3,3,'3_20200408_3.png',1);
INSERT INTO mytable(Id,UserId,Image,Day) VALUES (4,3,'3_20200408_4.png',1);
INSERT INTO mytable(Id,UserId,Image,Day) VALUES (5,3,'3_20200408_5.png',1);
INSERT INTO mytable(Id,UserId,Image,Day) VALUES (6,3,'3_20200408_6.png',1);
INSERT INTO mytable(Id,UserId,Image,Day) VALUES (7,3,'3_20200408_7.png',1);
INSERT INTO mytable(Id,UserId,Image,Day) VALUES (8,3,'3_20200408_8.png',1);
INSERT INTO mytable(Id,UserId,Image,Day) VALUES (9,3,'3_20200408_9.png',1);
INSERT INTO mytable(Id,UserId,Image,Day) VALUES (10,3,'3_20200410_1.png',6);
INSERT INTO mytable(Id,UserId,Image,Day) VALUES (11,3,'3_20200410_2.png',6);
INSERT INTO mytable(Id,UserId,Image,Day) VALUES (12,3,'3_20200410_3.png',6);
INSERT INTO mytable(Id,UserId,Image,Day) VALUES (13,3,'3_20200410_4.png',6);
INSERT INTO mytable(Id,UserId,Image,Day) VALUES (14,3,'3_20200410_5.png',6);
INSERT INTO mytable(Id,UserId,Image,Day) VALUES (15,3,'3_20200410_6.png',6);
INSERT INTO mytable(Id,UserId,Image,Day) VALUES (16,3,'3_20200410_7.png',6);
INSERT INTO mytable(Id,UserId,Image,Day) VALUES (17,3,'3_20200410_8.png',6);
INSERT INTO mytable(Id,UserId,Image,Day) VALUES (18,3,'3_20200410_9.png',6);

18 rows affected

ImgCount	userId	ImageDay1	ImageDay6
1	3	3_20200408_1.png	3_20200410_1.png
2	3	3_20200408_2.png	3_20200410_2.png
3	3	3_20200408_3.png	3_20200410_3.png
4	3	3_20200408_4.png	3_20200410_4.png
5	3	3_20200408_5.png	3_20200410_5.png
6	3	3_20200408_6.png	3_20200410_6.png
7	3	3_20200408_7.png	3_20200410_7.png
8	3	3_20200408_8.png	3_20200410_8.png
9	3	3_20200408_9.png	3_20200410_9.png

Warning: Null value is eliminated by an aggregate or other SET operation.