Still a bit of a beginner myself, but for what it's worth:
Looking through the sample, I can see that in Imaging.c there's a line in initGL that uses the image:
// Load image into texture
loadTexture("Image.png", &Input, &renderer);
So you'll want to replace this with whatever image has been selected.
NB the documentation on this sample does state:
// This is not a fully generalized image loader. It is an example of how to use
// CGImage to directly access decompressed image data. Only the most commonly
// used image formats are supported. It will be necessary to expand this code
// to account for other uses, for example cubemaps or compressed textures.
So you'll find it's not general enough to work with any image. Stick with uncompressed PNG files.
Hope this helps!