Reduce in-memory size of image info dimensions.

For my personal server,

Before:
Corpus stats: 149.884 MiB mem: 787886 blobs (88.853 GiB) (279308 schema (6073 permanode, 150557 file (111819 image), ...)

After:
Corpus stats: 145.172 MiB mem: 787886 blobs (88.853 GiB) (279308 schema (6073 permanode, 150557 file (111819 image), ...)

Change-Id: I47484e46b1eb70c8c4b5ee356cdcb0ceecde94e7