Looks like the design app losing images. Version: 3088. Mac OSX
Here are steps to reproduce:
1. create a project with bootstrap UI framework with cordova check the design checkbox.
2. Drag into the few field Graphic Buttons
3. copy into the www/images/path/image.png
4. check the box image src and select the path to that image. I also checked the box ID and gave it an id name
5. do some other things like adding some elements or change properties on the graphic button, change to the code and back to the design.
After a while you notice that the image disappeared from the app. Switching to code view it does not have the <img> tag anymore. switching to design mode the check box image src is unchecked! That is very annoying.
This is a bug in the current version (v.3022) of Intel XDK. I will escalate this issue to be addressed in a future release. I recommend manually coding your project as oppose to using the Design tool if you can.