If you use the Modal Popup (I think this is default), the description display is a configuration parameter option, and it will display when the image popup is displayed - but not during the slideshow.
If you use Slimbox, the javascript library uses the anchor tag details to populate its arrays for the image popup and slideshow displays; if you edit the anchor tag output for your images, the changes will appear in your overlay window.
You can accomplish this in /components/com_phocagallery/views/category/tmpl/default.php.
The original file includes (near line 120):
echo '<a class="'.$value->button->methodname.'"';
if ($value->type == 2) {
if ($value->overlib == 0) {
echo ' title="'. $value->title.'"';
}
}
echo ' href="'. $value->link.'"';
Manually appending the image description to the image title text in the title attribute of the anchor tag will affect the Slimbox display. Something like this works for me:
echo '<a class="'.$value->button->methodname.'"';
if ($value->type == 2) {
if ($value->overlib == 0) {
echo ' title="'. $value->title.' - '.strip_tags($value->description).'"';
}
}
echo ' href="'. $value->link.'"';
Make sure you strip the tags, the description value that is stored includes html tags that will ruin your display. Also note that I did not test this extensively. It shouldn't cause issues for non-Slimbox overlay windows, but I can not guarantee that.
Hope this helps - good luck.