Shapeways Image Popper has few if any options other than size and base or baseless. See my shop I have same issues.
FAQ-https://www.shapeways.com/creator/image_popper#
Any objects you need for jewelry like bails or mounts etc you must add to your image file and edit first.
While you can upload several sizes the Image Popper does allow sizing and thickness. If your image is not "water tight" you will have to add a base to keep the parts touchings so they can be printed. Specify no base if so needed and image support itself.
Loose parts like this will fall apart.
No, you would have to add your own 3D models and images/pictures for your shop. I had to start learning a free 3D tool!
You also cannot download your Image Popper Model when it's completed...I wish you could!
Size- this does effect cost. So a S,M & Large size is a good idea for pendants, charms etc.
Lines- This will vary by material and design complexity/orientation. A good guess is stay above 1mm unless you have tested and worked with one specific material in your design. Also Image popper will not scan fine lines or they appear weak and jagged.
Image Popper is dumb and will allow many models that will not actually print it most materials.
Lets post some successes and some issues for more help!