pictures blurry

As you can see from my webpage below my headers and banners are looking blurry.
https://www.wackynation.com/our-events/reverse-running

I don’t have photoshop so I have been using ‘paint’ to create all my banners/headers. I have now read that to get good quality images with text on my wordpress webpage, that you are better to save in GIF and you cannot do this in ‘paint’ without losing quality.

Can anyone suggest a free either downloadable or online programme where I can create banners and headers that look good in wordpress. As you can see I am not looking to do anything too complicated.

Thanks