Since we're just pulling numbers out of our butts here, I would prefer something simpler. All these fancy constants make it look like we actually know what we're doing, and (IMO) make a simple calculation harder to understand.

How about this:

// It is very unlikely that a JPEG file will have a file size that is more than half a byte per pixel.
// There is some fixed EXIF overhead that is the same for small and large pictures, however
var MAX_IMAGE_SIZE = this._pictureSize.width * this._pictureSize.height / 2 + 10000;

This would actually be better if we called it MAX_FILE_SIZE instead of MAX_IMAGE_SIZE