Ordering Gallery images by custom order
string str = "";



There's no sense in assigning str to an empty string just to over write it two lines later. This would be much more terse. 

ImageManagerV1 imgMgr = new ImageManagerV1();
string str = imgMgr.galleryOrder(LocalConfig.ImageManagerAuthCode, Convert.ToInt32(hdnListingId.Value), "");





str.Replace("seq[]=","-").Split('-');



Why not just split on "seq[]="? The Replace feels unnecessary here. Actually, the more I look at this, why are you splitting? Split returns an array, but you're not making an assignment here. I really think you need to decide what this line is supposed to do. 


str = str.Replace("&amp;", "");
if (str == "") {
    for (int i = 0; i &lt; nodelist.Count; i++) {
        str += "seq[]=" + i;
    }
}



Two things. As a maintainer of the code, I've no idea why you're stripping out the ampersands. A comment here is in order. Secondly, this loop needs a StringBuilder. StringBuilder is a mutable string of characters and thus, more efficient in this situation. 


string[] strNew = str.Replace("seq[]=", "-").Split('-');



Okay, now you can skip the Replace and split directly on "seq[]=". Speaking of, that string seems to show up an awful lot. You should replace it with a constant. 

string[] separators = new string[] {"seq[]="};
strNew = str.Split(separators, SplitStringOptions.None);



    for (int i = 0; i &lt; strNew.Length; i++) {
        if (strNew[i] != "") {
            str += "-" + strNew[i];



At first glance, I want to tell you to use a string builder again, but I honestly don't have the foggiest idea why you just split the string apart, just to put it back together. Forget what I just said about splitting, and just Replace("seq[]=","-"). Don't split. It looks to be unecessary.