"... so I could use a template like this: {{ImageSelector|season = 30|character = Claire}}. However, when I make the following template, it simply doesn't work"

This should actually work.

Can you link that template, I couldn't find it.

Also, for what I see, only "Category:Afbeeldingen: Seizoen 30" is populated with images so far, so until all your images are properly categorized to such season-categories, DPL finds nothing to display.

Oh, sorry, I didn't actually save the template yet. I saved it now: https://nergensbeterdanthuis.fandom.com/nl/wiki/Sjabloon:ImageSelector

And yes, "Category:Afbeeldingen: Seizoen 30" is populated with images, because I want those images to display in the Gallery. Or do I misunderstand?

I set up this page: https://nergensbeterdanthuis.fandom.com/nl/wiki/Test to test the template btw.

Yeah, DPL can be tricky.

Guess you have to use its parser function for the parameters to work. But then the gallery template won't work anymore, so you have to hardcode it.

This version works:

{{#dpl:
|category = Afbeeldingen: Seizoen {{{season}}}
|namespace = File
|titlematch = %{{{character}}}%
|format = «gallery widths="180" spacing="small" bordercolor="white" position="center"»,%PAGE%\n,,«/gallery»
|count = 16
|ordermethod = title
|order = ascending
|allowcachedresults = true
}}

/edit:

Or use the gallery template like this:

|format = ²{GalerijPersonage¦,%PAGE%\n,,}²

Thank you soooo much! It worked!