Try it. ;)

Other than subscribing or using image editing software to put text on an image and then uploading, how?

Look at the image description. :D
But I'll tell you Thursday.

Okay yeah I uploaded this pic and then made invisible text to say the same thing so that it would show it in the image description.

The problem with that is that people who like a particular image such as First World Problems, 10 Guy, etc., will go to the image and look through the other captions for the image and upvote or comment on some of THOSE. But if you upload an image (or if you use Add Image), Imgflip considers it a new IMAGE, so it doesn't get placed with the others. The result is that in general only a few people see it when it is in the "Latest" category, and it can last there for less than 30 views--so few, if any, upvotes.