Google找了半天找了个可以用的软件。写在这里，说不定还有用呢
XNB EXPORTER
XNB 4.0 Exporter released on 2011-06-04.
XNB 3.0 & 3.1 Exporters released on 2012-11-16.
Thread and readme updated on 2015-06-17.
Created by Nullable (nullable.se)
Distributed by Demian Wright (www.dataorb.net)
XNB Exporter is a simple and crude program that converts .XNB files to .PNG images. It‘s not fancy but does the job. Please notice that you cannot convert .PNG images back to .XNB files with this program. In theory these exporters should work with any XNB images that have been exported with XNA Game Studio 3.0, 3.1, or 4.0 provided you use the correct exporter. For Terraria use the XNB 4.0 Exporter.
1. Place "XNB 4.0 Exporter.exe" into your "Steam\steamapps\common\terraria\Content\Images" folder.
2. Open "XNB 4.0 Exporter.exe".
3a. Type the name of the .XNB file into the text field without the .XNB extension and press "Go for it!". For example to convert "Armor_Arm_1.xnb" file, you would type in "Armor_Arm_1".
3b. Alternatively to convert all available .XNB files in the same folder where the executable is, simply leave the text field empty and press "Go for it!".
4. The specified .XNB files will now be converted to .PNG images and saved in the same folder where the original files are with the same names.
Attempting to convert an XMB image that is the wrong version will lead to the program crashing.
DOWNLOAD XNB EXPORTER FOR XNA XNB 3.0, 3.1, & 4.0
VIRUSTOTAL SCANS
I had minor malware problems (No files were infected.) with my website early last year which prompted the removal of the download link in this thread around the same time. Those problems have since been fixed.
Website | XNB 3.0 Exporter.exe | XNB 3.1 Exporter.exe | XNB 4.0 Exporter.exe
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用法 ：
1: 将程序放到xnb文件夹中;
2:运行程序;
3a:输入文件名，不带后缀（.xnb）;
3b:要转换全部的xnb文件请留空不输，直接点“Go for it”;
版本不对可能使程序崩溃，
所以请分别尝试;
下载地址：点点我！
本文地址：
http://www.cnblogs.com/Hslim/p/5172253.html
原文地址：
http://www.terrariaonline.com/threads/xnb-exporter-extract-graphics-from-terraria.16058/
seo: xnb,xna, xnb to png,XNB EXPORTER