As you might have noticed, due to engine limitations, screenshots saved through Steam will only show the CG images, omitting the textbox and backlog.
But it's actually very easy to overcome this limitation, with a workaround I'll explain below.

I, nor any of the companies involved with this game, shall, for no reason, be held responsible for any side effect of using this guide, including but not limited to community bans.

I wouldn't exactly call this a guide, but I hope someone will find it useful regardless:

• Step 1: take a screenshot like you normally would, this will act as a "placeholder" - for the sake of this guide, we will call it "Steam screenshot";
  
• Step 2: take a screenshot with an external tool of your choice (Windows's print screen function should be enough) - this will be the "Windows screenshot";
  
• Step 3: modify the Windows screenshot as needed;
  
• Step 4: replace the Steam screenshot with the Windows screenshot, either by copying the content of the second into the first, or by changing its name to reflect the first's;
  
• Step 5: upload the resulting screenshot;
  
• Step 6: enjoy your new screenshot with text!

That's it, a bit of hard work just to upload a screenshot, but not too difficult.
Keep in mind a few things:

• firstly, since you're taking a screenshot with a third party tool, it will capture everything that's currently shown on your desktop - most notably your mouse cursor, the Steam screenshot thumbnail, chat/achievement notifications, eventual FPS counter(s), and so on;
  
• due to the same reason, you can have a final screenshot with a different size than the original 1280x720, but make sure to keep at least the original ratio (16:9) to avoid distortions;
  
• depending on how you save your screenshot, the thumbnail might still show the original unedited one (although the actual screenshot will be fine);
  
• last, but not least, even if you upload a screenshot with a higher resolution, it will still be upscaled from 1280x720, so its quality won't magically get better.

Maybe you can circumvent other kinds of restrictions, but be aware that with great power, comes great responsibility... ( ͡° ͜ʖ ͡°)


Lucia knows what you are gonna do, you pervs :3

Feel free to translate this guide into your language, Japanese and Chinese would be especially useful.