I use picsart to edit my photos and also thumbnails for my youtube videos. It has a variety of backgrounds and themes to choose for your pics. Some designs are very good but they are not free. You need to purchase it.
It has also features like collage making, adding magic effects, transparency, pip photos etc. The collage maker grid provides hundreds of free templates. There are various tools to build your images effectively. You can adjust size, rotate, make cutouts, crop, stretch, clone, brighten images and do many more things using this picsart.
It also includes customizable brushes, layes and professional drawing tools.The stickers are abundant in quantity and can be useful whenever required. I like the feature of creating own stikers.
The only drawback is it takes more space and time to load. But it is the best photo editing app as it has all in 1 featues for editing.