Me and my husband separated for four months and he went to Denver with his brother. After that we got back together.

Well I found out that he danced with other woman and took a picture with his brother's girlfriend, like kind of sexy, knowing that woman is married with his brother, and have a baby me.

My husband and I are together now, but how can I forget about how these past pictures hurt my feeling?

Sorry this hurts your feelings and I understand why it would. He was separated from you at the time at least. Maybe he was only trying to make himself feel better and maybe he didn't feel a thing for her especially since he's back with you. I'm not sure if I understood everything clearly. You have a baby with him? If yes, this is not the only reason why you are together or back together? Have you talked to him about this other woman? Maybe it will clear things up a little for you and I hope it does. Do try communicating with him about this if you haven't already. I am hoping that it really may have meant nothing. Was he drinking at the time? Was he hurting over that fact that you two were separated at the time? Paint the picture more if you will.
Hope it all works out and feel better!

Legally separated or just living apart?

In time you will realize that what he did while you were separated was just having a good time without you. Time is the key to understanding your own feelings and dealing with them honestly, and maturely. Express the hurt calmly and hopefully he will reassure you and explaining his hurt at the time also.

No reason to think the worst, and keep the wounds about the separation, or the events that lead to him, open, is there?

To make a fresh start means you have to make a clean start. You can't look back on anything prior to the date you got back together. It's the only way you are going to make it work.