In order to use your own picture as a post image or as a graphic object, you have to upload it through the Uploads section of the Graphics Editor. You will easily find it the very left of the menu left to the art-board in the Editor.
Being logged in the Editor, hit Uploads tab on the left, then click green Upload button to bring an image from your computer to the platform. Such formats as .png, .jpg and .jpeg should be used. The file will appear in the art-board immediately after uploading. Now, you can simply drag and drop uploaded images to the Editor.
All the imported files will remain in your Upload section forever. And when you want to delete a certain image, just click on the cross sign.