Hello and welcome to the forum!
First, you're not going to accomplish your goal w/ images that are 50 MP in size, i.e. GIGANTIC! The resolution on the larger iPad Pro is 2732 x 2048, which when multiplied gives a total of 5.6 MP - iTunes needs to down-size those images to fit the iPad's screen resolution.
Second, you can try to resize the images on your computer to say '5 MP' and then try an upload (probably will not satisfy you?) - there are other ways to get images onto your iPad, e.g. take a look at my post #6 in this
THREAD - might provide slightly improved resolution - not sure?
Please let us know your results - many ask these questions. My bottom line is to achieve the resolution of a 50 MP image requires a much larger screen that is also high pixel density. Dave