Formal:
* A stunning image.
* A captivating photograph.
* A breathtaking vista.
* A work of art.
* A magnificent portrayal.
Informal:
* A gorgeous picture.
* A beautiful photo.
* A lovely image.
* A sweet picture.
* A killer shot.
Descriptive:
* A picture that takes your breath away.
* A picture that evokes strong emotions.
* A picture that captures a moment in time perfectly.
* A picture that is full of life and color.
* A picture that is simply mesmerizing.
You can also use more specific adjectives depending on the picture, such as:
* A vibrant picture
* A serene picture
* A dramatic picture
* A whimsical picture
* A nostalgic picture
The best way to say "a beautiful picture" is to choose the words that best describe the image and the feeling it evokes.