1. There is no "global parameters". Each snippet call uses just the parameters that are set in the call and default values if parameter not set in the call. Default parameter values can be found from the wiki page. If you’d like that certain parameters always use a certain value, you could use the snippet default properties system in MODx manager.
2. Watermarking and everything else related to the uploaded picture modifying (masks, drop shadows, resizing) is done on upload. If you change these parameters you need to upload the pictures again.
3. For the question about users to select from a dropdown box to which gallery they’d like to add pictures, do you mean frontend or backend? I’m assuming frontend.. so just list the gallery documents with wayfinder or ditto for example and do a custom template to the snippet so that it outputs the results as a form with dropdown box in it. Then make the go button of the form to look at the selected value of the dropdown box and move to that document id. Or better yet if you have a document what goes straight in to the management view of the gallery (eg. manage_gallery parameter used).
4. No, there’s no validation.. One way would be to make the pictures to be automatically hidden (with the maxigallery hide field) and do not let other than admin users to edit that setting.. this would probably require some work to get it going.
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