That's not a development question, but I'll see if I can help: there are a huge number of applications available to create images, and to a large part it depends what kind of images you want to create! For icons, I use Greenfish: Greenfish Icon Editor Pro 3.6 - Greenfish Software Official Website
] which is free, but for everything else I use PaintShop Pro (now a Corel product) - except for specialist stuff when Painter gets dragged out (again Corel).
But there are also Paint (free with Windows), Gimp (Open source but I don't get on with it) and the "market leader" Photoshop (Adobe) with a price to match.
There is a lot of price / capability balancing you need to do there, and cheap / free stuff may not suit your way of working, while the commercial apps may discourage due to the price and learning curve (PSP and PS both have a huge number of features, and you need to understand how they work to do much that is useful). While Paint works, it's pretty basic and simple to learn - but get fancy and it fails dramatically!
Up to you: there isn't a "single answer" to this - it's down to your personal circumstances and preferences!