I have recently discovered Inkscape and have been playing with it, so far I like it.
What I am learning:
1. I use Inkscape it’s easy to use and free!!
2. Use Layers! I break down a drawing into areas and assign several layers to each. For example: For a hat I’ll have 2 or 3 layers: 1 hat background layer, 1 hat detail layer, 1 hat shading layer. That way if I don’t like the color or placement of the hat details (lets say flowers) I can move them around without disturbing the hat background.
3. When uploading to DT: 1st JPEG image then, upload EPS (thanks Robert) (or other vector file) image in the area.
NOTE: Wait until JPEG image is in the pending files (Thank you Maen) to upload vector file
4. Not sure how long a vector file takes to be approved (mine haven't yet). So, be patient.
Photo credits: Rocio Veltman.