I agree to the above, if you're using an image that you saved off of the web.
If you want to reply using a template that's not in the Popular Memes Tab, but still in the directory, then try this. It is actually faster than Googling an image and saving it because chances are that the image you want is in the directory.
1. Click Meme Comment
2. Click Add Image (third button across the top of the template that appears)
3. There is a box that says Search Images. You put in a title or description of the template that you want. If you don't find what you're looking for, try clicking Include User Templates. If you do that, you'll get a pop-up that basically says that you may get some NSFW images. Click OK and you'll get a lot of results.
4. Click on the image that you want
5. Resize it properly
6. Add something funny and relevant
7. Wait for the upvotes to roll in