Painting does not always need software like Photoshop to be installed on your computer. A great website with many high-end features including layers and filters is sumopaint.com

Try this short exercise

1. Paint a background layer

2. On the file menu choose 'Import to layer' and 'from my computer' and open an image

3. Choose the magic wand tool and click on any area of solid colour to remove then press the 'Delete' key to clear it.

3. Right click the background layer and choose 'duplicate layer' from the menu

4. Move the copy of the layer up so it is above the imported image

5. Use the erase tool to make a hole in the top layer to reveal part of the image

6. Save to 'my computer' (SUMO if you may edit again or JPG for a file you can share with others)

You may end up with a picture something like the following

Make sure you pay attention to which layer is active when you are working. Almost anything you do only applies to the currently active layer. (Top layer in the layers image above)