There is 2 ways of doing photomontages, traditional and digital. I am going to look at digital first. First you take images which capture different parts of the object. Put the images into a folder. Open Photoshop. Go to file and go down to automate. Click photomerge and a menu should come up. In the menu go to use then click folder. Find the folder you put your images in. When the images has been uploaded and click ok. On the side of menu there are different layouts try each one to see which works best for you. That is how you do photomontages digitally. There is another way of doing photomontages digitally. Open a new document in photoshop. Place the images until they line up and save it. To change the background colour you select the paint bucket, choose your colour, select your background layer and then click on the background on the image.