Could you please try putting the code below to "Code injection in footer" field in site settings?

<style>
.single-media img {
    max-height: 600px;
    object-fit: contain;
}
</style> 

 Thank you so much. 
The size is good. But now I have two images.

Also, the payment widget has disappeared. I inserted the code on footer before the widget code. Was it correct??

https://tomoko.npub.pro/post/note1eeeqf6f4asnxfwv8c06sjfzywhajk9jhlhr4jqe07pa2azcjccuse2r32u/ 

 Ok this will take some time to resolve, need a new configuration option  

 No worry at all. When you have a chance to look into it 😉 
I will go with the original size. It is not a big issue. 

Appreciated.  

 No the size issue is not related, you can keep smaller sizes 

 Though it shows two images if i insert the code. 
I prefer to have one image. 

 hum, I deleted the code but it shows two images....
I am sorry. I am bit confused. Please ignore the previous note.