Hi jnskrft,
Welcome to Affinity Forums
There's a non-destructive object based crop Tool you can use to hide the parts you don't what. Just select the image and the Crop Tool and drag the handles to hide the parts you don't want visible (this will add a vector mask to the image - expand the image layer in the Layers panel to see the nested mask). If you still want to get ride of the hidden parts, right-click the image layer (not the mask layer) and select Rasterise.
