Theme Horse

@miyukisaegusa You need to set the featured image for every gallery post. Did you do that? The recommended size is 474 * 342px. So, when you set the featured image size of 600*460px then automatically the 474*342px size image will be created so Image size should not be the issue. If you are still facing the issue, then you can send us your temporary username and password via https://www.themehorse.com/contact-us/free-support/ ( NOT HERE ) and I will look into it.