Of course, we know about image layering in photoshop (the little box down on the lower righthand corner of the screen)–you have your background, then assorted other layers–images, text, other images, other text–all overlapping each other. When you put a layer on top of another one, the topmost is the “most visible”. If, like Duchamp, Warhol, Man Ray, or Buñel, you want to make it so said layers form a dialogue with one another, or, to be less theoretic, make it so they don’t cover each other up, simply adjust the opacity of the images.
This is your opacity meter. It is right above the layer box.
You can’t change the opacity of the background, I don’t think, so it’s better to start with a canvas and paste your first image into the first layer above the background to maintain complete control.
You’ll see as well that photoshop messes with the “lightness” of the picture when you make it more transparent. To counteract this (if this isn’t your desired effect) go to image > adjust > brightness/contrast and mess with that, or, get crazy with it and go to image > adjust > color balance; or even image > adjust > hue/saturation. All three of these menus should help you get some pretty psychedelic/chaotic effects. Remember, after you’ve adjusted the image one time, you can always go back and make the effect more extreme: the sliders work only with the current image, not the original image. So if you push the contrast all the way up, and it’s still not enough for you, tell the menu OK, and go back and push it up again. Same works for color saturation and hue too. Adjusting is the best part of this experiment.
Here’s an example:
Try also adding effects to either one of the images to really mess with things. If you already know about layer masks, you can also use them to create really interesting vignettes and floating objects in your layered image. Have fun.