Step 3: Find images / print / cut
In your image editing software set up a new work space with the same dimensions as standard printer paper, A4 [8.5"x11"]. The inside diameter of the bottle caps were measured at about 25mm [1"]. In the image editing software I set up a simple grid template of circles with the same diameter as my bottle caps. Accounting for printing margins I got about 8 circles across and 11 deep.
Each image I found from the internet was copied onto the circle template. I've provided the sheet I used, feel free to use it as-is or as a template for your own images. Here's the full-size image.
When your template is full of images print in colour and then cut out each circle. After cutting place one circular image cut-out in each bottle cap.