OK, here is the step-by-step method.
Basic idea is simply to add the appropriately themed slideshow icon with a link to the slideshow. To do this, the HTML templates need to be modified and because the added HTML has 'variables' (i.e. {SLIDESHOW_TGT}), the appropriate values have to be set before being passed to template_eval(). A side effect is that the global variables $lang_img_nav_bar and $THEME_DIR are now used in functions where they weren't before.
All modifications are in include/themes.inc.php (version 1.4.3).
Around line 291 (when assigning to $template_album_list), find:
<span class="alblink"><a href="{ALB_LINK_TGT}"><b>{ALBUM_TITLE}</b></a></span>
Add the following after the above line:
<a href="{SLIDESHOW_TGT}" class="navmenu_pic" title="{SLIDESHOW_TITLE}"><img src="{LOCATION}images/slideshow.gif" border="0" align="top" alt="{SLIDESHOW_TITLE}" /></a>
Around line 410 (when assigning to $template_album_list_cat), find:
<span class="alblink"><a href="{ALB_LINK_TGT}"><b>{ALBUM_TITLE}</b></a></span>
Add the following after the above line:
<a href="{SLIDESHOW_TGT}" class="navmenu_pic" title="{SLIDESHOW_TITLE}"><img src="{LOCATION}images/slideshow.gif" border="0" align="top" alt="{SLIDESHOW_TITLE}" /></a>
Around line 502 (when assigning to $template_thumb_view_title_row), find:
<td width="100%" class="statlink"><h2>{ALBUM_NAME}</h2></td>
Replace it with:
<td width="100%" class="statlink"><h2>{ALBUM_NAME} <a href="{SLIDESHOW_TGT}" class="navmenu_pic" title="{SLIDESHOW_TITLE}"><img src="{LOCATION}images/slideshow.gif" border="0" align="top" alt="{SLIDESHOW_TITLE}" /></a></h2></td>
Around line 1548 (in function theme_display_album_list):
global $CONFIG, $STATS_IN_ALB_LIST, $statistics, $template_tab_display, $template_album_list, $lang_album_list;
Replace with:
global $CONFIG, $STATS_IN_ALB_LIST, $statistics, $template_tab_display, $template_album_list, $lang_album_list, $lang_img_nav_bar, $THEME_DIR;
Still in function theme_display_album_list around line 1596, add these to the assignment of array to params:
'{SLIDESHOW_TGT}' => "displayimage.php?album={$album['aid']}&pos=1&slideshow=".$CONFIG['slideshow_interval'],
'{SLIDESHOW_TITLE}' => $lang_img_nav_bar['slideshow_title'],
'{LOCATION}' => (defined('THEME_HAS_NAVBAR_GRAPHICS') ? $THEME_DIR : ''),
In function theme_display_album_list_cat around line 1632, find:
global $CONFIG, $STATS_IN_ALB_LIST, $statistics, $template_tab_display, $template_album_list_cat, $lang_album_list;
Replace with:
global $CONFIG, $STATS_IN_ALB_LIST, $statistics, $template_tab_display, $template_album_list_cat, $lang_album_list, $lang_img_nav_bar, $THEME_DIR;
Then add the following lines around 1682 (at the end of the assignment of array to params):
'{SLIDESHOW_TGT}' => "displayimage.php?album={$album['aid']}&pos=1&slideshow=".$CONFIG['slideshow_interval'],
'{SLIDESHOW_TITLE}' => $lang_img_nav_bar['slideshow_title'],
'{LOCATION}' => (defined('THEME_HAS_NAVBAR_GRAPHICS') ? $THEME_DIR : ''),
One last function, theme_display_thumbnails around line 1718, find:
global $CONFIG;
global $template_thumb_view_title_row,$template_fav_thumb_view_title_row, $lang_thumb_view, $template_tab_display, $template_thumbnail_view, $lang_album_list;
Replace with:
global $CONFIG, $THEME_DIR;
global $template_thumb_view_title_row,$template_fav_thumb_view_title_row, $lang_thumb_view, $template_tab_display, $template_thumbnail_view, $lang_album_list, $lang_img_nav_bar;
Add the following lines after 1775 (end of assign of array to params):
'{SLIDESHOW_TGT}' => "displayimage.php?album=$aid$cat_link&pid={$thumb_list[1]['pid']}&slideshow=".$CONFIG['slideshow_interval'],
'{SLIDESHOW_TITLE}' => $lang_img_nav_bar['slideshow_title'],
'{LOCATION}' => (defined('THEME_HAS_NAVBAR_GRAPHICS') ? $THEME_DIR : ''),
And you're done!
I'm not an HTML/CSS person, so if there are better placements for the icon, please feel free to enhance.