Within the dashboard, navigate to 'Media Library' section
Click on the area below the 'Search' area marked as 'Drop files here or select files from your computer'. A popup opens on your screen.
Click on 'Select Files from Computer', which would open a computer system folder to select a file for upload. You can also select and upload multiple files at once, or upload an entire folder.
Once the files(s) or folder has been selected, the resources should start uploading and you should get an ImageKit.io's Image URL that can be used immediately.
You can also upload images to ImageKit.io's media library programmatically. Read the Upload API documentation here.