First, go to the Editor of your landing page. Once there click on the image icon from the top of toolbar and just drag and drop for the image gallery to open up allowing you to upload your image .
Once the gallery appears, select the image you want to include in your landing page and click on "Select". The images you upload will be available for your future landing pages but keep in mind that they can't be deleted. As long as your images aren't larger than 500KB you can upload as many as you want.
We recommend not using heavy images as to avoid the landing page from uploading slowly. If you want to edit the image you can do so either using the properties panel or the little arrows on each image.
Extra Tip: Use good quality images and avoid stock photos on your landing pages to get higher conversions!