To specify a custom image to be shared by the "share" button:
Unfortunately, this is not possible. You can only customize the URL and the language; see the documentation.
Edit: this actually is possible, see BrettJ's answer.
To use a custom image for the "+1" button:
Because Google wants the +1 buttons to look consistent, it's not possible with the official Google +1 JavaScript. However, with a trick such as this one it's possible to customize the image.
The trick is in setting the CSS opacity to 0
, like this:
.myCustomButton *{
/*Hide real +1 button*/
opacity:0;
-ms-filter:"progid:DXImageTransform.Microsoft.Alpha(Opacity=0)";
filter:alpha(opacity=0);
}
.myCustomButton{
display:inline-block;
/*The width and height of your image. Not bigger than the Like button!*/
width: 10px;
height: 10px;
/*Your image*/
background:url(gplus.png);
}