In the context menu of the layers there is an option to change the transparency of an image layer (or rasterised vectors) interactively. This option can be activated from the toolbar and also from the Layer menu.
Icon on the toolbar.
Example of extension of transparency and results.
When the tool is activated, a dialog appears with a scroll bar that lets you change the layer transparency interactively. It must be said that the interactive transparency tool has its impact over the transparency display of raster layers.
