SELECT 
  PI.product_id, 
  F1.id AS file_id, 
  F1.path AS file, 
  (
    SELECT 
      PI1.alt 
    FROM 
      `cscart_products__images` PI1 
    WHERE 
      PI1.original_file_id = F1.id
  ) AS file_alt, 
  F2.id AS big_file_id, 
  F2.path AS big_file, 
  (
    SELECT 
      PI2.alt 
    FROM 
      `cscart_products__images` PI2 
    WHERE 
      PI2.watermark_big_file_id = F2.id
  ) AS big_file_alt, 
  F3.id AS small_file_id, 
  F3.path AS small_file, 
  (
    SELECT 
      PI3.alt 
    FROM 
      `cscart_products__images` PI3 
    WHERE 
      PI3.watermark_small_file_id = F3.id
  ) AS small_file_alt 
FROM 
  `cscart_products__images` AS `PI` 
  LEFT JOIN `files` AS `F1` ON `F1`.`id` = `PI`.`original_file_id` 
  LEFT JOIN `files` AS `F2` ON `F2`.`id` = `PI`.`watermark_big_file_id` 
  LEFT JOIN `files` AS `F3` ON `F3`.`id` = `PI`.`watermark_small_file_id` 
WHERE 
  `PI`.`product_id` = 26538

Query time 0.00059

Result

product_id file_id file file_alt big_file_id big_file big_file_alt small_file_id small_file small_file_alt
26538 323970 https://s3.timeweb.cloud/482029f9-prod/products__images/26538_e6eac1f7bcd1_processed_lot_100200_20171129163556__G6A0115.JPG.jpg 323974 https://s3.timeweb.cloud/482029f9-prod/products__images/26538_3c9322e1b071_processed_lot_100200_20171129163556__G6A0115.JPG.webp 323978 https://s3.timeweb.cloud/482029f9-prod/products__images/26538_736cab55b927_processed_lot_100200_20171129163556__G6A0115.JPG.webp
26538 323982 https://s3.timeweb.cloud/482029f9-prod/products__images/26538_dc2b72026d08_processed_lot_100200_20171129163616__G6A0197.JPG.jpg 323987 https://s3.timeweb.cloud/482029f9-prod/products__images/26538_10ebe40abbc6_processed_lot_100200_20171129163616__G6A0197.JPG.webp 323990 https://s3.timeweb.cloud/482029f9-prod/products__images/26538_a9145999404b_processed_lot_100200_20171129163616__G6A0197.JPG.webp