I am a designer by trade so I created this card in Photoshop. However because not everyone has access to Photoshop, I decided to put together a tutorial on how to make this Warhol effect in Pixlr Editor, a free web app that is pretty awesome. If you are a Photoshop user, most of the steps are pretty similar, so this tutorial is still relevant. For any experienced Pixlr users out there, tips and tricks on techniques I may have missed are welcome.
Step 1: Setup Image Document
Make a new image (file/new image) and set it to 900x900 (this is 3" at 300dpi, which is ideal print quality). I intended to have a 7" square card with four images, so this was as large as I needed. Make yours whatever size you need, at 300px per inch.
Copy and paste the image you had opened into your new square document.
Resize it to fit the space (edit/free transform). It's OK to size up a fair amount. When using free transform, hold shift to keep your aspect ratio the same. You can resize by dragging from the corner, and if you move your cursor slightly off the corner you will get your rotate option. I resized then rotated slightly.