You didn’t tell us the format used to save your signature picture. Many formats do not handle transparency, e.g. JPEG.
Sending the image to the background as suggested by @LeroyG is one solution but it is afterwars rather difficult to select the image to move it to optimal position.
I prefer to pre-process my signature picture in a graphics application like GIMP to keep only the strokes of the signature (this can be done with the “magic wand”). When having stripped all “noise” around my signature, I make sure there is no background and I save it .png.
With such a picture, I can anchor my signature as usual To paragraph, position it with the controls in the dialog and , most important, set its wrap mode to Through. With this mode, the signature is above text and, if color differs from printing text, it is obvious it was written above the text, as if added at last moment (which is not the case if it is sent to background).
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