1.18 added a lot of new image metadata properties (Exif and friends) that are now extracted that weren't previously. Please run refreshImageMetadata.php after the 1.18 deployment, which will go through every image and re-extract all the metadata.
Also, the script outputs the name of any image that had a larger img_metadata field before the refresh relative to after it. This could indicate a bug in new img_metadata stuff since almost all of the old properties are still extracted, so the img_metadata field should either stay the same or increase in size. (There is one exception to this, but it should be very rare). It'd be great if stdout of that script could be redirected to a file and posted somewhere, so I could investigate any cases of the metadata decreasing in size.
The command to be executed (for each wiki, **with HHVM** and not PHP 5):
hhvm refreshImageMetadata.php --force --mediatype=BITMAP