First, an image has to be uploaded to the correct gallery. Make sure the image is related to the gallery you're uploading it to, or it'll likely be deleted. If you're intending to use it as a third party host (using the URL to post the image on other websites), or to show off your image, KYM's not really that great of a place. ImgBB's good for third party hosting, while Memebase or 9gag would be good fits to show it off on.
Second, images are uploaded two ways: from your computer, or through a URL. In the article's gallery, you click on "Add an image," opening the uploading page. You can click either "Computer" or "URL" to choose. If you click "Computer," click "browse." Which opens up the uploading window. Here, you select the image from its file location on your computer. If you select "URL" instead of "Computer," you will need the image's URL to paste into the box. The URL must be the exact image's, not merely the regular webpage the image appears on. You can do this by right clicking on the image and selecting "Copy Image Location" before pasting into the box on the uploading page.
Third, Tags are keywords that are used during searches to help users find images. Tags help users identify the content of images, much as a word search helps them identify words on a page. KYM has no direct guidelines for tags (unfortunately), but generally, you want to identify the image type (comic, illustration, photo, etc.), characters that appear in it (Monkey D. Luffy, trollface, etc.), memes or references relating to the image (Loss, Doge, Shawshank Redemption), and general concepts the image contains (comedy, anger, satire, etc.). It's also good to provide a source for the image, so that the person who created it can be properly credited.
