Welcome to the forum jb310.
The highest allowed resolution (2400DPI in your case) is set in Preferences/Preflight Verifier.
As you have seen, you can just ignore it as it's just a warning but 2400DPI is way higher than you'd normally want (unless, for example, you're printing onto very high quality paper with a very high quality printer). This article http://www.vsellis.com/understanding-dpi-resolution-and-print-vs-web-images/
gives a decent basic (but not exhaustive) explanation of DPI/PPI.
Without seeing your results it's difficult to know what to suggest. "The lines have extra bits" doesn't give us a lot to work with.
Are you sure that it's Scribus where the problem lies? Trying to display such a high resolution image on a 70-90PPI screen will always cause problems and your PDF viewer may be having difficulties. Basically it's trying to fit hundreds/thousands of image pixels into the same screen pixel. It might not always correctly guess which one to use.
You can force Scribus to lower the image resolution by setting the "Compress Text and Vector Graphics" option in the PDF Export dialog and setting the "Maximum Image Resolution" to something like 300/600DPI.
Even better would be to find out what the maximum DPI you need is - your print shop will be able to tell you, or your printer manual will tell you - and scale the images accordingly in something like GIMP. Having the DPI way too high is just wasting machine memory and you might have trouble distributing the file if it's very large.