Tilda automatically adjusts picture size. But it's better to scale down big images beforehand as the scale-down algorithm of, say, Photoshop, is more perfect than the one on the server.
Best picture size
Best picture parameters are:
Format: jpg
Width: 1680px
Dots per inch: 72dpi
Color Model: RGB
Compression ratio: 10
For pictures illustrating texts or in galleries: width - 1160px (12 columns) or 760px (8 columns)
