Solution that handles multiple .
s
var size = '-150x150';
for (var i=0; i < pp_images.length; i++) {
if(!pp_images[i].match(/\b(jpg|jpeg|png|gif)\b/gi)){
classname = 'default';
img_src = '';
}else{
classname = '';
img_src = pp_images[i];
}
//ex: img_src = a.b.png
var src = img_src.split('.'); //ex: ['a', 'b', 'png']
src[src.length - 2] = src[src.length - 2] + size; //ex: ['a', 'b-150x150', 'png']
src = src.join('.');//a.b-150x150.png
toInject += "<li class='"+classname+"'><a href='#'><img src='"
+ src + "' width='75' height='75' alt='' /></a></li>";
};