How to say something covers all the view up to the horizon line?

An idiom which can help here is as far as the eye can see.

The wheat field stretches as far as the eye can see.

TFD(idioms):

as far as the eye can see
Extending to the farthest possible point, as of that which one can see in the distance. Sometimes used hyperbolically, humorously, or sarcastically.
I love this part of the country. The green hills stretch out as far as the eye can see.

Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

as far as the eye can/could ˈsee
to the horizon: There was only sand as far as the eye could see.

Farlex Partner Idioms Dictionary © Farlex 2017

Because you are referring to a photo, you can say the field fills the frame:

What is the Frame and Why Fill It?

Here, the frame refers to the edges of your photograph or the edges of the viewfinder of your camera when you are shooting. The advice to fill the frame means to get in close, to make your subject a significant portion of the final photograph.
_{Composition: Fill the Frame}

This expression can be used to refer to something large (like a wheat field) or something small (like a single flower), as long as it fills most of the picture. This expression can also be used no matter if it's a photograph, drawing, or painting.