I’ve been using Photoshop for over twenty years but I’ve never got into using masks to select areas of an image, partly because I find it difficult to draw precise outlines with a mouse or a graphics pad. Now that there’s an iPad version of Photoshop I’m beginning to see the point of it. Masks are non-destructive, so your original image, in this case the highland cow, is still there if you decide you’ve erased too much of it.
The meadow is the river embankment at Skelton Lake, while to highland cow (or bull?) was grazing in a pasture at Middle Wood near Redmire Force on the River Ure in Wensleydale.