You can use matlab or python or some utility to convert each image into a vector of values and then load those in to simbrain. I wrote a small program to do this with bitmap images. It's in utils and is called PatternsFromBitmap. But it's not a simple process right now.
We are working on an image processing utility for Simbrain, which will hopefully be in the source code by the end of summer. But it's hard to estimate when it will be in the main release.