Okey doke, as mentioned by JohnVV above, basically, use the trace bitmap in inkscape.
Take the image below, load it into inkscape.
Select all so the trace has something to work with.
Choose Path->Trace Bitmap from the menu, (shift+alt+B)
Now I’m not familiar with InkScape, but the following worked in this instance, but you could probably play with the edge detection options to get different results when working with colour images. So say selecting "edge detection in mode, rather than greys under multiples, may produce a different set of cruves, so do experiment.
Under Mode->multiple, choose greys
Click the live preview and you should see the image looking pretty much unchanged.
Click OK to trace it, once the OK button returns to normal, close the trace window.
You can prove the trace was successful by selecting the node icon and clicking on the image, you’ll see a bunch of curve points.
Now simply save this as an SVG.
In Blender, import the SVG, it will look messy, but it’s a whole group of curves.
I selected everything and scaled them by 10 and applied scale, simply because it was a better size to deal with.
I found in my case, “curve.001” contained the curve I wanted to work with, (curve itself was just the “background plane”).
Now you can convert this to a mesh, play about with the curve values or do whatever you wish.
Bear in mind, if you convert to mesh, I’d advise doing some clean up, such as going into edit mode, doing a limited dissolve then checking for areas where there are possible overlaps due to points in close proximity if you want to use bevels.
Have also attached my resulting Blend file so you can see the result I got.
CelticKnot_svg.blend (1.62 MB)