A small embellishment to GaryP's challenge.

Here is a template, which you can use to design and test your seamless tiling.
It only offers the steps to go from design to presentation for the Challenge but enough to take you into building usable seamless patterns for your websites.

Seamless.xar

Even if you cannot create an SVG, do submit your PNG version as even that can be used in a tiling; SVG is just making it crisper, smaller and faster to present.

Acorn