Icon in unlock screen and in login screen is of low resolution. See screenshot in attachment for screen of 1280 x 1080 pixels.
Distribution & Version: Lubuntu 17.10
Kernel: 4.13.0.16.17
Qt Version: 5.9.1+dfsg-10ubuntu1
liblxqt Version: 0.11.1-2
Package version: 0.9.3-1ubuntu2
lxpanel: 0.9.3-1ubuntu2
See also screenshot in other window:
This is a Lightdm issue, and that is not LXDE software. Please, report this to the Lightdm developers. Thank you.