I have two ideas for this plugin so this is intended to Daniel, awesome author of this BackWPup.
Exclusion of intermediate images
I propose that you add an option to exclude those intermediate images from backup and to backup only original, full sized images. One can later regenerate those intermediate sizes with some of available plugins.
List of intermediate images can be got by getting all attachments and passing array of sizes (got via
Conditional scheduled backups
Scheduling backup is a great thing but some sites don't have enough activity to be backed-up constantly and since backup can take a lot of resources, there could be an option to check if some conditions are met before backing-up is done.
Those conditions would be predefined and would center around several items that are included in backup.
For example, there could be check if something related to posts is done (new post is created, post is edited, trashed, deleted), media (new/edit/delete/trash), plugin (install/upgrade/activate/deactivate) etc.
BackWPup would know if something was done by hooking into related hooks (for example
comment_post etc) and then comparing time of last update with time of last action.
I could help with coding/testing this features if you need that.
Thanks in advance.