Modding

Hello everyone! I decided to create my own splash screens for loading scenarios, but I encountered a problem: what image format should I use? The mod uses the .dds extension, but when I rename it to .jpg, it opens, but when I take the same image and use the same extension in Photoshop, the game doesn't launch...

https://yadi.sk/d/n7vy51x03Yyfup

Can someone who has worked on this mod tell me how to reformat the standard graphic file?