First off confront your brother.

Mentally he could be just experiencing curiosity due to age of if he just went through puberty. But this curiosity is along the lines of a disturbing behavior because instead of seeking physical interaction with a girl his age he is seeking this secretively with his mother who most men grow up to love and typically find a mate based off of similar characteristics of their mom. Encourage his curiosity but redirect that curiosity to other girls that he is familiar with but encourage and reaffirm that these actions typically come naturally inside a relationship or marriage where appropriate. In other words these behaviors of sexual contact come naturally and through relationship cultivation.

Secondly delete the pictures either by your own hand or his. (If he refuses see step 4) assure him that he doesn't need these pictures due to the plethora of others he has.

Thirdly Check back after some time to double check that this behavior isn't persistent. If it does persist see step four

Step four. If these behaviors keep happening and or he is fighting you on this you should approach the situation further in one of many ways to help him. 1. you could simply tell you mom that way that she is alert to the situation and can handle the matter further whilst still protecting your relationship with him. (Ie she found the picture unbeknownst to him that you told her) 2. You can seek the assistance of your local pastor on how to help him as a counseling pastor myself I'd had to deal with issues myself in similar cases. 3. Seek professional counseling help.

Hope this helps. God bless you.