To take a screenshot, simply press the "Print Screen" button on your keyboard. You will receive a message in the chat window that says "Screen Capture OK". To find your screenshots, open your La Tale folder and open the ScreenShots folder. This folder is typically found in C:\Program Files (x86)\La Tale\LaTaleClient.exe, but it may reside in another location if you have changed the installation directory or are playing from Steam.
If you would like to change the file type of the screenshots you take, click the Menu button in La Tale and then select Options > Settings > Misc. You can choose between .jpg, .png, and .bmp file types for your screenshots.