What pre-processing of the image is needed before feeding it into the convolutional neural network?
If you have a trained network that accepts input of 100 * 100 you can either scale your image accordingly or slice it into smaller parts. 

Scaling the image

Scaling your image down is really straight forward. It will lower the information and might distort it too if your aspect ratio doesn't match. In your case it is a little off, but most networks are trained with a little distortion.

You will have to ask yourself if the details you are interested in are still discernable, and whether they (still) match the examples the network is trained on.

Slicing smaller parts

If you are interested in features rather than the larger scene, you could slice your image into bits of 100 * 100. I recommend you use a sliding window, wherein you tile the subsamples as if they were shingles. That way you avoid that a feature is only availble in bit's and doenst get recognized.