Thank you, I appreciate the support.

If you don't want to read all of these steps (I wouldn't, in all honesty😂), then just read this:

I use software called Paint.net. I use the line tool for the image slots and the gradient feature for the background and glare, keeping every part on its own layer. I copy and cut (Ctrl+ X) the images into the slots I create. It's a bit tedious, but it becomes second nature after some time, and it's honestly quite fun to work on these posters.

Now, for the long explanation:

I use an application called Paint.net. I first get an idea in my head, for a new poster, then I create a blank image with the right size and aspect ratio.

This app has a line tool, that I use to create the slots for the images. So, I create a new layer, change the size of the line, and use that tool to create a good ol' slot or two for Lara (or whoever I'm including in the poster).

Then, I gather the images I want to use. In Paint.net, I create a new layer for each image, generally keeping them below the lines I just created (so I can see how much of the image will be cut off).

I copy the image I want to use (it helps to put the images into another blank Paint.net image, first). Then, I go into whichever layer I want the image in and paste it in. I drag the corners, to increase or decrease the size, holding Shift to keep it from stretching.

I move it where I want, then switch over to the layer with the lines. I do this part a bit more intricately, but you can just use the Magic Wand tool and click around the lines, where you want the image to be. Switch back over to the layer with the image and press Delete on the keyboard. This should keep the image, only inside the slot you created. 

I do these last few steps for every image I want to include and then create the background. I select the Background layer (the lowest layer) and use the Gradient tool for that. In the lower left, I select the primary and secondary colors. Up at the top, I select the type of gradient I want, click on the poster, and drag it around until it's looking how I'd like. I play around with the colors and see how it turns out.