PicWall is an easy-to-use collage maker that allows you to insert and edit photos, text, and create custom backgrounds. In many ways, this app is much like Pic Collage. It is user-friendly and fairly intuitive. However, one feature I prefer on Pic Collage is the ability to keep your work automatically saved within the app. Primary elementary students have a tendency to forget to save their work. :) One of the biggest points you'll have to drive with students using this app is that they will need to save their completed work before creating a new one.
If you need students to create a poster, slides for a slideshow/movie, or a cover for a published piece of work, PicWall will meet your needs.
Feel free to view the video below. I created it with the intention of students learning to use the app. You can show it to your students through a projector or TV. If you are a 1:1 technology school, feel free to send it off to your class. Here is a QR code if you prefer to have students reach links through a QR scanner. (The video is embedded from Youtube.)