Reduced max image sizes

Estimated heap sizes of loaded bitmaps: (megapixel * 4)

Small: ~ 7,5 MB - 15 MB
Medium: ~ 11,5 MB - 23 MB
Large: ~ 19 MB - 38 MB

This values should be more realistic.
Merge pull request #63 from L-Henke/reducedImageSizes

Reduced max image sizes