Yes, check image dimensions again. There must be something wrong with your image's proportions. E.g., I couldn't create an image in GIMP with dimensions 595x842 pt, but only 595x841.9 pt, which is enough for proportions not to match. Check your bitmap editor's capabilities to create a document with exact dimensions you need.
The easiest way to correct the problem using Scribus instead of a bitmap editor might be as follows:
1. Create a new document in Scribus, dimensions set to: 595x842 pt.
2. Create a new Image Frame with the same dimensions and coordinates x=0 and y=0, as you did before.
3. Right click on the Image Frame > Get Image..., to import the image.
4. Right click on the Image Frame > Adjust Image to Frame. Zoom to 3200% and take a look at the right side of the image. There is a white line? Go to the next step.
5.a. In Properties palette, go to "Image" tab and simply untick "Proportional" under "Scale to Frame Size", so that the image fits the Image Frame perfectly. Or, alternatively, do that manually as follows:
5.b. In Properties palette, go to "Image" tab, click on "Free Scaling", uncheck "Keep X and Y scaling the same" (the small binding icon just right of "X-Scale" and "Y-Scale" fields). Now click inside the "X-Scale" field and, with Alt and Shift keys on keyboard pressed, use your mouse scroll to change the value. Increments will be very small and you'll correct the problem easily.
This usually isn't the best approach, because you're changing the proportions of the image, but in case of only fractions of point, it shouldn't be a problem.
As for the resolution, if your document isn't intended for press, but for on-screen reading only, you probably can ignore the warning. But for press, you'll need at least 300 DPI.