Here's a shortcut I came up with...
1. Make a new image (I used Microsoft Paint to do this) and set the width to 1000.
2. Fill it in with all black (paintcan).
3. Then set the height to 1.
4. Save it.
5. Go to Meme Generator and upload it, naming it (I used "single black line") and sharing it (you can't download it from Search, unfortunately).
6. Once you've done that, it will be in your My Templates, and you can Add Image below it, using any image available on Imgflip.
Happy memeing, my friends!