I think it would be helpful to know what steps you took and in what order. Because I'm not sure why you would want to edit a JPG in Inkscape. Afaik, it can be done, although I will admit that I don't know what issues might be involved. However, I'm not at all clear what you're doing. Here's what I understand so far, and if you could correct it, I think we'll be able to sort out the problem.
1 - You got the image from jupiter, saved as JPG, then opened in Inkscape.
2 - You also said you changed it from XCF to JPG in GIMP. How did it get to be XCF? After you acquired from jupiter, did you edit in GIMP some and save in the native GIMP filetype XCF? Then still in GIMP, save as JPG and close; then open the JPG in Inkscape?
3 - After you opened the JPG in Inkscape, you said you resized it. At any time, did you use Trace Bitmap (from Path menu) to convert it to vectors? Or did you just add the vector Text on Path, on top of the JPG (but not on a new layer)? If so, this may be the heart of the problem.
4 - When you finished with your Text on Path, and saved in Inkscape, did you choose JPG from the dropdown list in the Save As dialog? Or did you just click the Save icon?
5 - In one place, you said you saw the red X and "Linked image not found" when you tried to save in Inkscape. But in another place, you said you saw it when you tried to open it again later in GIMP. Did you see it in both places, or have I misunderstood you?
I think the trouble may be that you're thinking Inkscape and GIMP do the same thing, but have different interfaces (GUIs). (And I could not agree more; I love the Inkscape interface; it's why I've gone to so much trouble to learn about vector graphics!) However, they are very different! GIMP is similar to most other graphics program, in that it is a raster type editor. Raster means that a pixel in the editor is equal to a pixel on the screen (or monitor), so that whatever attributes you have assigned to each pixel in a raster type program, are displayed in each on the screen. However, Inkscape is a vector type graphics program (SVG means Scalable Vector Graphics). Unfortunately, I can't explain vector grphics very well, except to say that they don't recognize pixels (except as a unit of measure). But here's some info from Wikipedia, and you can do your own research to further understand it, as well:
http://en.wikipedia.org/wiki/Vector_graphics.
Anyway, I suspect you tried to save your JPG as an SVG, which won't automatically happen. So if you can clear us up on each step you took, and in what order, I think we can help you sort it all out
