Photos, stickers and text all have an opacity setting. This setting refers to how much of what lies behind the object can be seen through it. For example, a sticker set to 50% opacity will show 50% of the sticker and 50% of whatever is behind it. In the example below, the penny sticker on the left has no transparency while the one on the right has 50% transparency.
To adjust the transparency of an object like a photo or a sticker you must first click on the object to select it. Then click on the opacity icon on the toolbar above the workspace. This will reveal the opacity slider (See below) which will let you adjust the degree of opacity. The degree of opacity can also be adjusted by entering the desired number into the input box to the right of the slider, or by clicking on the + or – on either side of the slider.