First off, install GIMP,
already linked it to you few posts above. It's a cool software, much better than a Photoshop. Why? It's free to use and it has ~80MB installation, so you can get it easily.
I'l assume you want the result like the picture above ( cool in-game pic ) so I'l explain you how to do that. First, go in-game. Under the "Controls" find the "User interface". There, you'l find control called "Disable HUD". Add a key to it, preferably one that you will not accidentally press while playing. Now, when you're ready to take a screenshot, hit the "Disable HUD" button, and if you have time, wait for few seconds so all server messages disappear from the screen to get a clear picture (or disable "Show joining players" and "Show leaving players" in the Launcher ) . Take it with Print-Screen button.
If you had no time to wait for a perfect shot, and there's a nasty text, message or something in your screen you'd not like to be there, GIMP has a tool called "Clone tool". Use it by clicking on it, then while holding Ctrl, click on part of the background that is exactly the same as background below the text. Clone it above the text. Takes time to figure it out, but I've managed to remove stuff from in-game screens perfectly, so it is possible.
The result is
this
Now, to add some cool color to it, GIMP offers another tool. Go under "Colors" menu and click on "Colorize". After moving the Hue to 285, adding a bit of Lightness and Saturation, you get
this
GIMP offers tons of filters too, you can find them under "Filters". The section I'm mostly using is "Artistic". Subtly adding those effects really make FL screens look nice. But hey, I won't explain everything to you, I'l just say - go play with them, you'l figure it out.
Edit: Right, few more things. "Crop Tool" for cropping the image, "File" > "Export as" for saving the image. Make sure you turn it into .jpg format by just typing it under the Name, as it'l lower the size of images greatly. Upload it to
Imgur or some other web service, resize it using the DiscoveryGC forum photo resizer - "Insert an image" and write new resolution. This way, the quality remains perfect, while GIMPs resizer messes it up.