You can check out the options documentation
Taken from that page, below excerpt shows how to customize url, text, image_url and raw_image_url properties. (You just have to override the getter methods.)
// Share buttons
//
// Available variables for URL:
// {{url}} - url to current page
// {{text}} - title
// {{image_url}} - encoded image url
// {{raw_image_url}} - raw image url
shareButtons: [
{id:'facebook', label:'Share on Facebook', url:'https://www.facebook.com/sharer/sharer.php?u={{url}}'},
{id:'twitter', label:'Tweet', url:'https://twitter.com/intent/tweet?text={{text}}&url={{url}}'},
{id:'pinterest', label:'Pin it', url:'http://www.pinterest.com/pin/create/button/?url={{url}}&media={{image_url}}&description={{text}}'},
{id:'download', label:'Download image', url:'{{raw_image_url}}', download:true}
],
// Next 3 functions return data for share links
//
// functions are triggered after click on button that opens share modal,
// which means that data should be about current (active) slide
getImageURLForShare: function( shareButtonData ) {
// `shareButtonData` - object from shareButtons array
//
// `pswp` is the gallery instance object,
// you should define it by yourself
//
return pswp.currItem.src || '';
},
getPageURLForShare: function( shareButtonData ) {
return window.location.href;
},
getTextForShare: function( shareButtonData ) {
return pswp.currItem.title || '';
},