From your screenshot it seems to me that you are not displaying the pasted images at their original size. So the problem is actually that you are displaying scaled raster images, not missing anti-aliasing.
yEd is not an image manipuilation application and does not offer sophisticated scaling algorithms for raster images. If you want to display scaled images in yEd, you need to scale these images in a dedicated image manipulation application (such as MS Paint, Adobe Photoshop, the GIMP, etc.), import the pre-scaled images into yEd, and display the imported images at their original size.