is it cheating?

Easiest way to look at "what is cheating" is:
If you have to explicitly hide anything from your partner then you are lying to them and if you are lying to them then you are in addition "cheating" them. In the case of sending nudes if your partner is okay with it? sure go ahead i mean you've got his/her permission why not? But just because you can do it doesn't mean you should. An honest to god relationship isn't something that is fleeting.