Some software will allow you to remove the background temporarily, but will add a black background to hold the size coordinates.
To get around that, I use one of two methods.
1. I use the edit function inside the built-in image viewer. How you do that is open the image and click the edit button. If you are using windows it should be in the topleft.
That should take you to the editor page. Click the background icon. On windows it is a person with a striped background.
The computer should auto select the background. If it didn't, you turn on the background brush tool and fill in the areas you want removed.
When you have the entire background selected, click the remove background option.
Click save>save as copy>png to preserve the alterations.
NOTE: This only works on still images.
2. Use the 3D Keyer in DaVinci Resolve
Open the film you want to edit in the fusion page.
Press CTRL+shift+space. In the window that appears search for "3D Keyer" and double click it to add it to your workspace.
If it didn't automatically, connect the white square on MediaIn to the yellow arrow on 3D Keyer and the white square on 3D Keyer to the yellow arrow on MediaOut. Next, select the 3D Keyer and click and drag your mouse on the viewer to draw a blue line on the image/video to remove  the background. You will see the background turning black. When you have selected everything you need to, release your mouse. Even though the background looks black, it will show through anything you put behind it. This works of photos and videos, though it works best with a greenscreen or bluescreen.
I hope this helps.