This article explains how to upload new images to your image library. Recommended icon sizes: Large icons 32x32 px and small 16x16 px.
Go to the ui tab and click add image and write a name for your image.
Click add image data. Right click the image area and select load.
Choose the image you want to upload and select open. You can upload several image sizes to the same image. Save on main save icon or CTRL-S.