If (for example) the image size arrays in your properties.php file are:
1024w x 768h
782w x 481h
163w x 92h
62w x 62h
132w x 140h
102w x 102h
42w x 42h
782w x 481h
999w x 260h
and if you are using the slideshow_dissolve gallery without editing it in any way, then the width will be 782px, and the height will be cropped (not resized) to 481px.
p = proportional, c = cropped
Line 14 of your slideshow_dissolve/index.php file would read:
$gallery = get_gallery(get_mediarss_url('gallery=' . $pg_id . '&thumb=62c&full=782c'));
The '782c' indicates 782px wide, height cropped (to 481px).
Look at the third image in each of the two galleries here and here.
The first (ribbon_fullsize) reduces the height to 481px (in which case the width becomes proportional to its height [481/782]*height , in which case you would optimise your portrait images assuming a finished height of 481px if the array selected in the properties.php file (via line 14 code in your index.php file = '&thumb=62c&full=782p') was proportional, 782w x 481h.
The second (slideshow_dissolve) crops the height of the image to 481px, assuming a finished width of 782 px if the array selected in the properties.php file (via line 14 code in your index.php file = '&thumb=62c&full=782c') was cropped, 782w x 481h. In this case, as the image is being cropped (not resized) you would optimise you image to fit width 782px, and a selected area height of 481px. The cropping is indiscriminate, and (presumably) crops to the middle.
EDIT: In either of these methods, the portrait images suffer a little in comparison to the landscape images. If you wanted to present them equally, you have two choices (or is it one choice between two options?) - either:
Have two galleries, one for P and one for C, - you would need to add an array to the properties.php file favouring portrait orientation, and add that option to your chosen gallery index.php file, or:
Create the display area of a single gallery for both with an equal height and width (i.e. 782px x 782px), and again you would need to add an (proportional) array to the properties.php, and add that option to your chosen gallery index.php file.
In each case you would also have to adjust the stylesheets that apply to the chosen gallery.