Here are the main guidelines for the logo image, which you can set in Theme Options » General tab.
No matter the dimensions of the image you upload, the theme will always limit the height of the logo to make it fit into the header. In Nayma, that limit is 60px.
To get the best results, upload an image that has at least an height of 60px (or a bit bigger), but never smaller than that.
If you also want to upload a version suitable for retina devices, then the size should be twice as big as the regular one.
We recommend using PNG with transparent background.