I've delayed my response in the hope of being able to offer a bit more technical detail, but in the absence of a perfect response I'd like to go ahead and post a few suggestions. Please forgive me if I end up needing to revise any of this [likely].
Updated info on preparing images is here.
Make sure your design is at least 150 DPI. You could create the pattern repeat on your own computer and then upload a large file equal in size to the amount of fabric you wish to order. For example, the fabric we will be using is 44" wide (112 cm), so if you wanted to order a yard you could create an image that is 42" (the printable area) x 36" (or close to that). The file size limit is 25 MB, so if your design is large you will almost certainly have to use the JPG rather than the RIF format.
You can order a swatch (8"x8"), a fat quarter (18"x21") or any multiple of a linear yard (up to 8 yards continuous). We will always recommend that you order a swatch of your design before ordering a larger quantity. That will give you a chance to examine the colors firsthand to make sure they printed as intended.
When you upload a design to your Spoonflower account, you will be able to create a pattern by tiling the image, using a half-step or half-brick repeat (which staggers the tiles), or by mirroring your image. In the case of tiling, in order for the pattern not to appear to be composed of a lot of individual rectangles, the design you upload will need to be composed so that the left side of the design 'joins' to the right side, and the top of the design 'joins' to the bottom. If you have a textile design program or Photoshop expertise, you can probably do this pretty easily. For the rest of us, however, we plan to post tutorials to help you through the process of creating patterns on your own.
File Types / Color Profile:
You will want to set up your files in RGB profile with 8 Bits/Channel (rather than 16), and to save (and upload) them as .JPG, or .PNG files with flattened layers. RGB standard is the ideal profile for printing.