A free app for Android, by photoeditor.developer.

Photo Mixer is an easy-to-use photo editor app. 

It allows you to combine multiple photos in a single image file by arranging them in a grid – which, to put it more simply, is just another way of letting you create a collage. 

The process of creating a collage in this app starts by providing you with a set of template options – 

You choose the template by considering the number of images you want to combine in a single 4X3 or 16×9 – or whatever standard or customized dimension is going to meet your requirement. 

When you are finished with the combining process, you can then customize the image by injecting text, or stickers, or effects to your personal liking.