Question Need help finding the name of this image overlay software

[freshprynze]

I found this old tutorial on YouTube and it uses a specific image software that the person used for the tutorial but the image software was not mentioned, he was able to overlay an image over the model and then changed the opacity of the picture to be able to see what he was doing to the model.

I am aware that we can use image panels to achieve a similar result but I would love to have the option of using this image software as well, if anyone happens to know what it is I would really appreciate it!

Note: the attached images show what the image overlay looks like and how they were able to change the opacity. Again, thank you!