Welcome Lucienne!
Oh wow - that tree trunk looks perfectly, totally realistic!! Did you draw that originally in Inkscape?
When you say you sometimes use imported JPGs for backgrounds, do the JPGs already have text on them? Or do you add the text later? If they already have text on them, then you need to be very careful not to scale the imported image.
That text looks pretty sharp to me, as long as I'm looking at the full size. If I zoom, it gets blurry. But that's a known problem. Zooming or scaling any raster format (PNG, JPG, GIF, TIFF, etc.) image will get blurry.
If you need the PNG or JPG to be bigger (or smaller), you should scale it in Inkscape, and then export again.
Or is there a more subtle problem which I don't recognize?