Lack of User Account Interface Manager in Xfce Desktop Manager

Except for the profile picture, what you want to do is normally done through the "Users and Groups" dialog. If for some reason it's unavailable through your GUI, it can be run from the command line with the command system-config-users. If the command's not found, you should be able to install it from your repo.

As far as I know, no one's yet gotten around to adding a profile picture setting feature to "Users and Groups". To change the picture, make the picture a png image (other types may work as well), name it .face (with no further extension), and put it in your home directory (i.e. ~/). If the image is too large or not equal in height & width, Xfce shrinks the image as needed to fit. I'm not sure if there are any size limits on the image.

To change the image on the login screen, you'll need to provide an image (with an ordinary name and extension) that's accessable by LightDM, and then set the User image in the LightDM GTK+ Greeter settings to that image. The image can be put in /home if your user home directory is encrypted, and the permissions may need to be set to make it readable by LightDM. You'll need to install lightdm-gtk-greeter-settings from your repo if the Greeter settings item isn't available in Xfce's Settings.

I originally picked up info on changing the profile and login screen pictures from the video How to set an account image in XFCE, with relevant stuff really starting at about 1:20.