I am not able to extend the image to the edges of the paper. How to extend the edges of the image to the edges of the paper?
First, go to the layers panel. Right click export, save as. Then in the window 'Save Raster Layer as...' select under extent 'current layer extent.' Read the dimensions. Go to the composer to make a layout. Fill in the dimensions correctly in under 'item properties, extents'(tip: look a rulers). Now go in 'item properties, position and size.' Change the page size. Somehow it worked for me.
Change the scale until your data fills the whole map-canvas.