I have checked in
content->design->configuration->Mytheme->Product Image Watermarks
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I think it is bug in Magento 2.3.0 because when you try same with Header logo image, it is not getting uploaded and thats why not showing old uploaded image there.
In vendor/magento/module-catalog/view/adminhtml/ui_component/design_config_form.xml change fileUploader to imageUploader in line number 21
You need go to: Content > Design | Configuration > Select Store View > tab Product Image watermarks.