I have no experience with Prism library, but let me comment on the things you brought on the table:
- Image files are located on the project root folder (App/App.PlatformName/**.png_or_jpg) which I believe should be put in another folder, specifically App/App.UWP/Images/*.png_or_jpg
If you question this, be prepared to answer the question "why"? Can you explain a clear benefit like "there are so many images among other files in that folder that we start to loose oversight"? Or is your main argument something like "at my former employer, we always separated such files"? The latter is a pseudo-argument - if that is what you have in mind, better hold back with any critics.
Moreover, there will be some effort to change access to the current location in the code base, and the software will not only have to be changed, but also reviewed and tested after such a change. Be prepared to explain why you think the benefit will justify that effort, and be prepared to suggest a way which keeps the effort for this change low.
- Image names are hardcoded as string literal everywhere. I think they should be put in a single file/class which contains all the image names as constant string and this should be generated by a script that triggers every pre-build.
Same as #1. Moreover, putting all names into a single class file is not necessarily a good idea, since it creates a dependency to a single point. Whenever that file is recreated or touched, this may trigger a full rebuild of the application. So if you want to suggest this, can you explain to the team for sure why this drawback is really worth the hassle?
- Lazy loading a Command, more specifically, a DelegateCommand is over engineering. The type is not even resource intensive to be a candidate for lazy loading.
Maybe that's true, maybe you haven't fully understood the reasons behind this implementation. In case you have the task to work on a command's implementation, and the lazy-loading makes the change more complicated, then it is perfectly justified to ask the team if they agree if you remove the lazy loading to make things simpler. But you should approach this with a mindset of "I guess it is not needed, but I could be wrong, so I will test this change thoroughly" and not with a mindset of "this lazy-loading looks like crap, lets get rid of it and hope nothing will break".
- Consider the code below: ...
...
I believe ...
This is clearly something you need to discuss with someone who knows the code base better than you. "Believing" is not a good basis for a change which can have non-local effects. If you come to the point where you can honestly say "I have made a detailed impact analysis on this ...", then you have a way better foundation for changing this code. Moreover, if the code is working well now, and it currently does not cause any issues, and you don't have a task which requires to change something around or related to it, you should consider just to make an entry in your teams issue tracker to analyse or change it later.
- String literals are stored on a .cs files (noticed the S) instead of a single .resx which would make localization a nightmare task although there's really no task for it
If there is no requirement for localization, and maybe there will never be for the specific application, you better hold your critics back here. The "Just in case" mindset is the most direct path into overengineering and wasting time for no apparent benefit. As I wrote above, "I was used to this from my former employer" is a strawmen, not a valid argument. Moreover, I guess it is debatable if localization would become "a nightmare", as you wrote, or just a little bit more work in case it will ever become a goal.
In short, keep in mind refactoring and restructuring code is not an end in itself, it is a means to an end, and never comes completely for free. Whenever you do this, make sure you know why you are doing it, how the cost-benefit relationship will be, and which risks a certain change will introduce.