Option 1 - Adding a background colour

Step 1:

Open a design or create a blank canvas. Click on the colour palette in the panel found on the right of your screen titled Background colour ↓
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Step 2:

You can also add a gradient effect to your background by clicking on the gradient toggle found in the colour palette. Click on either of the buttons on the slider below the gradient toggle to add the two colours of your preference ↓

Step 1:

Click to select the canvas, which will then open up the options on the right of your screen.

Step 2:

Click on Add Image and close down to wallpapers or narrow your search by entering the keywords in the search bar.

Step 3:

You can make adjustments to the overall appearance of the background on the edits panel.

Step 1:
Open a design or create a blank canvas. Click "add image"in the panel found on the right of your screen titled Background image ↓
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Step 2;
Once you have added your image, you can adjust the size and position according to your preference by clicking on reposition ↓

Step 3:

You can also make adjustments to the overall appearance of the image by clicking on Effects ↓

Step 4:
In order to remove or replace the image you just added, simply hover your cursor over the thumbnail of the background image and click on the Remove or Replace button that appears ↓