Even a clear picture, if it is small, will have a lot of jaggies in it and make it hard to render into a cuttable pattern. And sometimes I've run into high resolution images that would not convert until I reduced their size. Don't know why, but it's true.
If the case is a small image with jaggies, I use Inkscape to increase the picture size. The bigger image will have the jaggies kind of blurred, but smoother.
For blurry images, I usually use one of the edge detector filters in GIMP. Artistic/Cartoon is a good one, but there are a bunch of them.
For pictures that a too bright I use the Cartoon filter. Note that can add a lot of "freckles" so you may need to increase the value of Options/Suppress Speckles on the Trace Bitmap dialog in Inkscape.
For pictures that are washed out after Artistic/Photocopy, I go back to the original and increase Brightness and Contrast to about 50, and add edge detection.
As a last resort, open the image in Inkscape as Linked while it is still open in GIMP. If the Inkscape Trace Bitmap is not giving enough detail, go back to GIMP and trace some lines on the image with the pencil tool (e.g., outline the nose or around an ear etc.) then Save. The changes will show up in Inkscape and you can retry the Trace Bitmap. But don't overdo it - more detail does not always make a better image.