The default image type is .jpg (0.9 compression quality). You can change it to .png using appropriate option. To enable the snapshot button you need to set appropriate Nuevo option.
var player = videojs("my_player");player.nuevo({ snapshot:true });
var player = videojs("my_player");player.nuevo({ snapshot:true, snapshotType: "png" });
var player = videojs("my_player");player.nuevo({ snapshot:true, snapshotWatermark: "Domain.com" });