1. Unfortunately, the current implementation of custom fields doesn't support custom providers. All other functionality in images would work with a custom provider, though. If separation of images by some condition is the only reason for creating a provider, think of some other way of distinguishing between them (tags, categories, your own taxonomy).
2. You can filter items by the value of a dynamic field through dynamic LINQ. Here's an example (I have created a custom field with name TestField and type ShortText):
var images = App.WorkWith().Images().Get().Where(@
"TestField == "
" && ContentState == "
Note that you have to include the Telerik.Sitefinity.Data.Linq.Dynamic
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