Photoshop is still the standard in terms of Photo, image editing. But it is heavily paywalled. So if your looking to learn editing still's from the ground up , there are tons of free apps out there.

One of these apps is Photopea. With it's interface similar to old or classic Adobe Photoshop models, you can use it to learn, practice or build your skills in photo, image editing for free.

This is what I use for simple tasks like thumbnails for my content.