This is a setting that can be customized in conf/_advanced.php if one would want to add a custom image size. I am curious to know why this has to be done manually on this page and why its not something that can be done via a setting that can be stored in the database? Simply to make upgrading easy without loosing custom changes. I know upgrade_policy.conf can be set to ignore replacing this file during upgrades, but I am wanting to understand the thinking/methodology behind the way it is?
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You can use http://b2evolution.net/man/local_configuration for not losing your changes.
The original idea was to standardize on a few sizes and have most users use the same which was supposed to optimize skin switching. Not sure it passes the reality check :p