Use the global insert menu on top or the image option on the Elements panel on the left to insert the image . You can also shift + select to add multiple images at a time.
Once added, you can customize and edit the image using the contextual panel
After uploading an image (or selecting an image from a grid), the image panel will appear on the right side of the screen.
From the panel, you can manually adjust the pixel size of the image, as well as customize these features: