Is is possible to change the attributes of the thumbnail frame, when using frames? For example, to change the width of the frame, or change the thumbs from align=center to align=left? I couldn't find any information about this in the manual.
The way the thumbnail images are arranged on the index page is determined by the template design and by the settings for thumbnails used in Galerie main window: Number of columns and rows, whether or not in a vertical or horizontal frameset, at different horizontal or vertical distances, and with different frame sizes. Which settings are available, depends on the used templates. For instance, some template designs are incompatible with variable frame widths.
Make sure you use the full interface to have all possibilities available. (You can switch between full and simplified interface in the window menu.)