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I'm not sure how you would insert an image in the signature.
Hi,
In the signature edit window you should be able to switch the signature editor to rich text:
Click on the text "Switch to rich text"
> screenshot #1
Do you see the formatting bar when you do? (bold, italic, etc) > screenshot #2
The "insert image" icon is on the very right of that icon bar.
> screenshot #3
You may need to scroll the icon bar sideways, to the right, to see the insert image icon.