Images sources
http://i.imgur.com/PjlxnLw.jpg
https://ii.yuki.la/9/da/eeb731b72fb619eed28d6b266110cefa7e7436a639eab4d93182082e6fa30da9.jpg
https://ii.yuki.la/f/a5/5836bdef3f18de08612ebc8d65acda9b24d744d2bc4e2ebd7e436a75fda16a5f.jpg
https://i.imgur.com/dfjd18V.png
https://pbs.twimg.com/media/DIZgIXxVoAAaK6S.jpg

Go to your steam dirty bomb folder. \SteamLibrary\steamapps\common\DirtyBomb\ShooterGame\Content\WebAssets\images\play

inside the folder you should have 5 jpg files (and their sizes in pixels)

• tile-assaultcourse.jpg (330x250)
  
• tile-quickjoin-stopwatch.jpg (330x650)
  
• tile-servers.jpg (330x400)
  
• tile-quickjoin-objective.jpg (330x650)
  
• tile-tutorial.jpg (330x150)

1. make a backup of your \play folder (just in case)
   
2. take the image you want to make your tile with (ex: coolstuf.png)
   
3. open your favorite image modifier (paint / gimps / photoshop). and depending on what tile you want to modify, let's say the quickjoin-objective tile.
   
4. scale your picture file (coolstuf.png) to (in this case quickjoin-objective) 330x650
   
5. save it to tile-quickjoin-objective.jpg (convert to jpg if it's not )
   
6. replace your new jpg file in the \play folder
   
7. ENJOY

If you don't know something use google

Ps: make a backup of your modified files because they'll probably be erased after an update

http://steamproxy.net/sharedfiles/filedetails/?id=1104115339
http://steamproxy.net/sharedfiles/filedetails/?id=1121982768