Montaging is making new images by chopping up other images and putting them together in new ways.
Go look at dada montage by Hannah Hoch.
Gee Vaucher was part of the punk band CRASS. Go take a look at her work.
The Edinburgh Collage Collective
Brilliant collection of montage work.
Collection of punk artist's work.
You can "isolate" a picture, or get rid of its background using the mask tools in Pixlr or Photopea, but if the picture is well defined, tools like Remove.bg may be able to do it for you quickly.
The latest version of this fantastic image editor. Has fantastic tools for isolating a part of an image.