Decreased max image size
Removed unused imports
Added selectable maximal image size

Let the user select between three maximal image sizes in megapixel:
Small: 2 > X > 4
Medium: 4 > X > 8
Large: 8 > X > 16
Changed out_of_memory_message

Added the hint to select a smaller maximal image size.
Merge pull request #62 from L-Henke/directImageScale

Decreased max image size