How to upscale the quality of screenshots of the character CGs?

Does anyone have any advice on how to upscale the quality and sharpness of the character CGs from the gallery on iPhone? I’m not super tech savvy, so forgive me if this is a stupid question. 😅 I’ve taken screenshots from my phone of the artwork, but when I zoom in, the photos are blurry and not as sharp as I’d like them to be.