When choosing an appropriate picture for the logo to your login screen,
consider the following picture requirements:

All the major formats are supported: 
ANI, BPM, GIF, ICNS, ICO, JPEG, JPEG 2000, PCX, PNM, PBM,
PGM, PPM, GTIFF, RAS, TGA, TIFF, XBM, WBMP, XPM, and SVG.

The size of the picture scales proportionally to the height of 48 pixels.
So, if you set the logo to 1920x1080, for example,
it changes into an 85x48 thumbnail of the original picture.