@Boonthanom 

Such kind of issue arrives because one of the reasons mentioned here:

1. Permissions issue on your /media/ folder.

chmod -R 777 media/

2. Low PHP memory limit: (Increase it)

php_value memory_limit 256M or 512M

3. The GD library is not installed or configured.

4. Wrong attribute scope for small_image, thumbnail, base_image
Sometimes Magento processes images incorrectly, if they have scope=Store View

> In this case, open your Magento backend > Catalog > Attributes > Manage attributes.
After this, find image, small_image & thumbnail attributes.

> Make sure that all these attributes have scope = Global.

5. improper store wise static content command:

php bin/magento setup:static-content:deploy -f en_US(As per your store)