If you want to add your own photos to the game, you should be able to use Microsoft Paint to convert and resize the images and that software should be pre-installed on your PC.
To convert files:
1) Hit your Windows key and type "paint".
2) Open the file you downloaded (I assume you downloaded an equirectangular image using the search link I provide: www.jumblivr.com/photos)
3) Click the re-size button in the toolbar at the top
4) Select the pixels radio button
5) Set the Vertical to 4096 and Horizontal to 8192 (2048 x 4096 works well too and is recommended for mobileVR).
6) Click File -> Save As and change the filename extension from .jpg (or .jpeg) to .png.
To copy files on PC VR in Windows Explorer
1) Right click on your new file and select copy.
2) Click in the current path in the thin box above-left of your files.
3) Type: %localappdata%/VRnChill/Photos
4) Right click in the empty space in this folder and select paste.
The new photo should now be shown in the game after the standard photos.
I'll try and do a video tutorial when I have some time.
Thanks for playing.