Credits to @Kazuhito from his answer
Try the "Opacity slider" via RMC > Properties > Legend > Embedded Widgets in Legend > Available widgets
and edit the transparency
This is more a workaround but still can be useful:
Transparency/Opacity can be a tailored globally (all layers etc.) or individually by layer.
This way is by layer:
Layer Styling [Shortcut] is F7 in QGIS.
This can be docked in QGIS for quick access.
This then allows the user to pick the slider for any of the layers in the canvas including images/raster's.