There is a lot written here on monogamous people attempting to achieve a happy healthy relationship with polyamorous people. You might want to try a search in the tags for mono/poly. Its our largest selection under one tag. There is lots of interesting stuff to read.
I don't think that asking for pictures is something that could be an issue. Its the deceit and untruth by which it happened that is concerning. It seems concerning for her too. Yes, it could be considered cheating, but there is far worse. I would be taking this as a warning that you have some work to do to figure out how to make this relationship work and your relationship life in the future.
Personally I wouldn't settle for sucking up how I feel because of who I am with at your age. I wouldn't do it with out any other partners either. I do with the bf I have because he is mono and we continue to work toward some kind of future regardless. It seem slightly hypocritical of me to make these suggestions really as I do suck up a lot for him, please take it as me understanding where you are coming from.