This, unfortunately, happens because we are not yet at the stage where we have a database of your most chosen images as the popular images. So we are still working with Google's/ Bing's suggestions.
But, don't you have filters set up?
While we do have filters set up, some pictures still get through the net. This should be resolved soon.
Do you have any tips for when I am searching to avoid these...erhm...creative images?
As for inappropriate images in your custom search, the goal is for there not to be any, but just to be sure, make sure that you are searching very specifically. Learning "chicken breast" is awesome (and they are delicious, yum!) and very important for when travelling. When you want to add a picture for chicken breast, do not search for "breast", this might not please the Google/Bing Gods, and who knows what might show up. Instead, search for "chicken breast", fried Chicken filet", etc.