The origin of the images is not important, what matters is how you can get them on to the Nextion display. As far as I know the only way is to include them when you design the layout of the Nextion configuration using the Nextion IDE. There is no way to change them or upload them other than in the configuration.
Now you could say that as it is possible to change the images by editing the Nextion configuration in the IDE and uploading the change via the serial port then it must be possible to get another program other than the IDE program to do the same. I would agree with that, but I personally would not know where to start and I have never read anything by anyone who has done this.
What you see in the Nextion instruction set is what you can do. If it's not in the instruction set then you can't do it easily.