1) On a PC right click the image you want to use or touch and hold the image on a phone, there should be an option to download the image.
2) Use that image in the imgflip meme generator or wherever you want to post it. In the imgflip meme generator, you don't have to add captions for memes (but you do for meme comments).
3) Use the "Go to image page" button, edit the image title, add at least one tag. Another way to get to any of your image pages
imgflip.com/creations
4) hit the "Submit this image to the Imgflip community" button, pick repost from the dropdown list, check the "I Certify..." box, and hit the last Submit Image button there.