[ Need an app to do this job ]

[ring]

Okay. I need an app that you type a pixel resolution, and it makes a selection like this. Kind of like in preview, except I don't have to figure out the exact resolution through difficult scrolling efforts.

etc

 

[simsaladimbamba]

Gimp could do that, and in a way, even Preview could do that, as you can see in below video:

 

[ideal.dreams]

GIMP does all that and then some. Definitely the best free photo manipulation software out there.