This theme creates medium size images of 800x400 resolution by default. At this
size an image page should fit into a screen of 1024x768 resolution. You can
override this parameter by specifying "-medium ..." AFTER the "-theme ..."
option. There is also enough place for 5 columns at this resolution, I suggest
you try "-columns 5" too.

The theme also implies "-no_crop", you can override it with "-crop" AFTER the
"-theme ..." option.

If you have several portrait thumbnails, it may be disturbing that they are
smaller than landscape ones. Specify equal values for "-geometry=..." to get
around this. (133x133 gives nice results but it may be too big on the image
pages if the title bar, task bar, menu bar, tool bars, etc. are visible.)