Query using group_concat is returning only one row

When I try to use group_concat it just returns me one row and I do not understand why.

Because you have not used the GROUP BY clause in your query. When using aggregate functions like GROUP_CONCAT you need to tell the database about the column using which you want your data to be combined.

Currently your query is grouping all records and giving 1 record in the output.

If you add GROUP BY users.userid in the query then the records will be grouped by unique userid's. I updated your fiddle and it now gives 2 records: http://www.sqlfiddle.com/#!2/867f6/18

Please note: In standard SQL queries, columns listed in the GROUP BY clause should match the column in the SELECT clause (except the aggregate functions).

Just use group by clause in your_query

SELECT users.first_name, users.last_name, 
       users.user_id, projects.project_id, 
       projects.project_name, projects.project_time, 
       group_concat(images.image_path)
FROM users, projects, images
    WHERE users.user_id = projects.user_id 
    AND users.user_id IN (SELECT follow_user_2 FROM following 
                          WHERE follow_user_1 = 1)
    group by users.first_name
    ORDER BY projects.project_id DESC;

fiddle