In Photoshop (or the free alternative, GIMP) open up the images you want to combine in a bigger image.
To keep the scale of all the images the same when you paste them into your larger, containing image file, you should make sure they all have the same resolution (pixels/dots per inch).
To check the resolution of an image in Photoshop, go to that image window (Window then select image name) and then select Image, Image Size from the menu. You can change the resolution of your image here by typing in a different Pixels/Inch (dpi) and then clicking OK.
Work out roughly how big your containing image needs to be. If you are planning to print on a particular fabric, check the maximum printed width for that fabric.
Then create a new image that is the same width as the fabric, and make sure that it has the same dpi as all the images you are going to place within it. Select File, New from the Menu to do this: enter your image width, height and resolution (Pixels/Inch or dpi).
For each image that you want to include in your larger file, do the following:
Use the Move Tool to position your pasted image where you want it to go.
When all of your images have been pasted into the larger, composite image file, you might want to continue to position them. For each image that you pasted into your composite file, a new layer will have been created. To move a particular image, make sure that you click on that layer in the layers window (usually at the bottom right of the screen), and then you will be able to move that particular image using the Move Tool.
You can resize each separate image, also, by selecting the layer for that image, then selecting Edit, Transform, Scale, and dragging the corner of the image to resize it.
When your images are arranged and sized as you want them within the larger, composite image file, save that file.
Go to Create Custom Fabric to select and buy your fabric. You are welcome to send us your image file separately via wetransfer.com, dropbox, or email, if you prefer - you don't have to upload a file at our website to place your order.