Take a look at the following example in documentation:
Use a marker to specify the holes to be filled
How to make such mask quickly and interactively? I post here a function that allows to do that.
MakeMask[i_] :=
Manipulate[
Row[{Show[i, PlotRange -> Thread[{{1, 1}, ImageDimensions[i]}]],
mask = Image[
Reverse@Transpose@
SparseArray[# -> 1 & /@ Round[p], ImageDimensions[i]],
ImageSize -> ImageDimensions[i]]}],
Button["pasre mask", Print[mask]],
{{p, {ImageDimensions[i]/2}}, Locator, LocatorAutoCreate -> True,
Appearance -> Style["+", Red, 40]}, {mask, ControlType -> None}]
