Recently, I moved my website based on phpBB to a new web host. Then the pictures attached on the posts could show.
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Now the problem is solved and I share the resolution here:

I use FileZila to transfer the files. Now, I change the setting of the FileZila. It’s as below.

Default transfer type chooses “Binary”. Go to Transfers=> File Types and click to open the window. Then uncheck “Treat files without extension” as ASCII file. Then, upload and download files again. At last, refresh the case of the website and delete the Browser’s history. Now open the webpages, you can see the pictures.