Opacity 

To be able to see both images you will need to change the opacity of the top layer.

 This is at the top of the layer box.

 By doing that you can give this effect.

Layers 

to add a picture over another picture, will create a layer.

Here you can see all the levels in which the picture has. You can also delete and duplicate the levels by clicking the small icons at the bottom. If you hover over these Photoshop will tell you what they do. The eye icon allows you see the the layers you want, which is useful for when you want to go to a certain image.

Photoshop Workshop 3

Changing levels / brightness / contrast.

To change the brightness of a picture you could either click Image, Adjustments then select Brightness/Contrast. 

Once selected you can change the pointers to change the brightness to your choice.

Or you could click Image, Adjustments then select Levels.

Here you can also change the brightness of a picture by moving the pointers.

Action Photos

Adding Borders To Pictures.

To add borders to pictures, select the photo which you want the border for, click Edit and then click Stroke.

Choose the width of border, (the higher the number the thicker it will be).

For example, the chosen width of this border was 20 px.

Putting multiple pictures onto the same file 2.

To add the pictures to the same file, drag the desired image onto the file and choose how big you want the image to be.

To make all the images the same size, choose the size for the first picture, then each time you add a new picture, click Edit, Transform then Again.

Putting multiple pictures onto the same file.

To start, have a blank file open and the files you want to move onto it.

Once opened, go to the blank file and tick the boxes Auto-Select and Show Transform Controls. This will allow you resize the images once moved onto the blank file.