How do I remove the background for a product photo in my Shopify collection?

Hi, I'm starting up my first e-commerce store using Shopify. I tried removing the background of a product image that I found and when I tried to upload it, it was pixelated and I realized that what I did significantly brought down the quality. How do I get it the image quality to remain the same? Is there a recommended app for a new Shopify seller with limited collateral?