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Right click on the part you want to add the custom image and click either "Paste Image from Clipboard to BOM Item" or ""Add / modify Modify BOM Part Picture".
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Right click → Paste or use the Paste Image icon in the upper right corner.
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This will update the image for the line item for just this one part. If you modify in the Master Parts List, it will update the part for all future uses. See here for additional details.