how can I set the default wrap mode for inserting images in text, in Writer

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Open Styles and Formatting pane, then tab Frame Styles. Right click the style Graphics. From the list choose item Modify. In the tab Wrap set the desired kind of wrapping.

This setting is stored in the document. If you want to make it automatically set in all new documents, you need to define an own document template and make this setting there. Then define that this new template is your standard document template.