Does anyone know how I could make some seamless patterns for spoonflower.com as they have a new special on chiffon fabric and I want to design a paisley scarf...

So I did a fabulous paisley design as black outline vectors and I thought I could make a very cool pattern by doing the old "opposite corner swap" and finishing the drawing. That's when the top left corner becomes bottom right, top right becomes bottom left and vice versa then you complete the pattern and it should line up, look very cool and of course be seamless.

However, I couldn't cut, crop or slice the image (I hoped I could use that photo crop "enhance photos" or even "clip" tool) but it wouldn't let me unless I exported as a transparent png, at 300dpi of course, and then import it and crop it that way. The only problem is, that the swapped opposite corners wouldn't line up, so somehow after exporting and opening or importing the image it got skewed. I was looking very closely (at 1000%) but it just would not line up perfectly, and the whole idea of seamless patterns is that you can't tell where they join.

I know xara isn't pattern making software, but has anyone ever tried it? If so could you please give some suggestions?