I wanted to create still frames to be used in Cinelerra, working with SD material, but Gimp and Inkscape produce images that are 1:1 on pixel aspect ratio. Video frames however, use 4:3 or 16:9 pixel aspect ratio. I've searched and tried a lot of things before sticking to this recipe:
If you're working in 4:3, open a new image of 640x480 pixels in either Inkscape or the Gimp. If you're working in the Gimp, I suggest you at least double the precision (so 1280x960), and we'll resize it at the end, no matter what. In Inkscape, you can just increase the output resolution when exporting. If you're working in 16:9, have your image 865x480 pixels WxH.
Work with your favourite tool, viewing the correct output. Then export, or save at a higher resolution.
Last thing, from the ImageMagick suite, run:
convert filename.png -resize 720x480! outputfilename.png
This should do the job. Now import it in Cinelerra. You'll note the bad resizing algorithm used to display the images in the viewer and compositor. These artifacts should go away when you export to some file, and view in a real video player.