Show two number fields below the Theme image box on the folder panel, where the user can enter the required dimensions in px. If he leaves a field blank the corresponding image bound of the jAlbum Images / General settings should be used.
This has several advantages:
The most important advantage: the user gets the possibility to define the theme image differently for each folder. I think that is not possible if the dimensions are asked in the skin code, because you can’t save the dimensions per folder.
Is is easy for the users: the same UI for all skins and all relevant parameters are grouped together.
Less work for the skin developers and an easy interface for the skins, so it can easy be implemented in all skins.
If the crop focus tool shows a bad result, you can adapt the theme image size in the same panel to get it better.
But some skins do have already there own implementation for the definition of the theme image dimensions, so these two number fields should only be showed if for instance the skin properties shows something like:
or even better enable it per album via a variable in onloasd.bsh.
But the best solution is of course if jAlbum offers the possibility in the theme image cropping window to save the new settings locally or globally.