Move README images to img/ subfolder

hardware/Mechanicals-BTU-Mod/README.md

@@ -15,13 +15,13 @@ Designer: [George Bryant](https://github.com/gbrnt) with help and bearing advice
 This version was designed in Rhino and so isn't really editable for most people. It works well though.
 Print this version if you like seeing the exposed edge of the BTU.
 
-![A trackball with this mod v3 installed](example-v3.jpg)
+![A trackball with this mod v3 installed](img/example-v3.jpg)
 
 ### v4
 This version was redesigned in FreeCAD and the various dimensions are now editable in the FreeCAD file.
 Choose this version if you want the BTU to be covered, or to be able to edit the file.
 
-![A trackball with the v4 mod installed](example-v4.jpg)
+![A trackball with the v4 mod installed](img/example-v4.jpg)
 
 ## Buying the BTUs
 The particular BTU this mod is designed for is the Bosch-Rexroth R053010810, or KU-B8-OFK. A set of three is about 20-30 USD, depending on where in the world you are and where you buy them from. Here are some sources we've found so far:
@@ -56,4 +56,4 @@ Recommendations for installing these are in the [Ploopy trackball assembly guide
 ### Modifying secondary buttons
 There is a small tab on the secondary buttons which collides with the closest BTU. It needs to be sanded, filed or cut off. It is indicated in red in the image below:
 
-![Ploopy secondary buttons, with the tab on the end highlighted in red](trim-tab.jpg)
+![Ploopy secondary buttons, with the tab on the end highlighted in red](img/trim-tab.jpg)