i assume you are using the latest gimp ver 2.8.2.
If so, try the following as a test:
Start a new session from the File menu create a new image and set template as, for example say A4.
Then select 'Layer' from the main image menu bar and create a new layer. Do this say 3 times so that you then have 3 thumbnail layer boxes showing in the r/h layer brushes window. I assume that this is similar to what you have currently.
Now to set each one with a different picture, using the Windows File Manager locate an image picture file of your choice and then select and drag the image to one of the layer thumbnails.
Repeat this again twice, each time with a different picture and drag and drop that to each of the the other layers.
Hope that works for you.