Image Extension for mobile games - 2D Tower Defense

Hey everyone, i’m working on a indie game with a friend, i’m the artist and i have no idea of wich image extension use to preserve quality of my artwork…
I tried vectorizing on Adobe Ilustrator but Unreal is not compatible with .ai, .eps nor .pdf… I tried working on Adobe Photoshop and saving the artwork on .PNG image extension but it didnt work out… can someone please give me some “for dummies hints” ?

The quality i’m looking for can be seen in mobile games such as “Empire Warriors TD” or “Kingdom Rush series”

Thank you all!