The gateway to your app on all platforms is your app icon.
As a mobile developer you need to be familiar with all the app icon requirements for the platforms you are building for - most importantly getting all the required sizes for your app icons.
Typically you need to work with a designer to create your app icon and most designers nowadays are very familiar with all the sizes and requrements but every now and then you are only provided one image (and hopefully it is a high res image).
MakeAppIcon is my defacto go-to tool for generating app icons for all my platform needs.
They provide a free web based version of their product that takes a single high-res image - ideally 1536x1536 and you enter your email and voila a few minutes later you have all images for all mobile platforms in formats you can easily put in your project.
Now if you don't want to have to provide them your email, or are not the patient sort then I wholeheartedly recommend purchasing their desktop versions - available for both Mac and Windows.
You simply drag your hi-res icon into their tool and instantly have all the various app icons at your disposal.
So what are you waiting for visit MakeAppIcon today.
Note - I am not paid for any referrals - I just happen to like their product cause it solved a problem for me.