If you've used an Atticus-provided ornamental break in your book, the resolution of these images was created to be higher than 300dpi so that when they are scaled up, which authors do have the ability to do in Atticus, the image doesn't don't lose any quality. So this is likely why IngramSpark is giving you this 'warning' about image file resolution being unnecessarily high.
However, it should not prevent you from publishing your print book through IngramSpark, and they should still give you the option to proceed past the warning. And after extensive experimenting by our team, we haven't experienced (nor heard any reports from other Atticus users) that their ornamental break images being higher than 300dpi caused any issue in printing quality through IS in any way, so you should be just fine to proceed as normal!
This is a common comment from them and they are simply comments, not necessarily errors. You should be able to continue through the process and your book will print perfectly well! We have many, many authors using Atticus files with Ingram and these comments do not in any way impact the quality. Of course we do always recommend ordering yourself an author copy to proof, but that is a best practice recommendation regardless of any comments or notes.