Let's say M3TJ20 has a texture that stores the Fog of War of a level on a per-pixel basis. That's something that would be needed to be saved and loaded again later. To do that, export your texture like so:
texturename.copyToImage().saveToFile("filename.png");
And then load it again with:
sf::Texture newtexture;
newtexture.loadFromFile("filename.png");
And like Limeoats said, do not save a texture in your database if it is going to be the same every time the user runs the program. This should only be done for user-made textures. Also, the user will be able to open the file and edit it. You would need to encrypt it somehow if you don't want the user doing this.