@lucoxox - I honestly think this is one of the things where you need to pick your battles - they are not harming the baby by changing him, and he honestly won’t remember not liking having his clothes changed - and stressing about this is only going to make you feel bad, whereas if you can ignore it and not let it bother you, it will take away a source of stress from your life.
It may be that, in the future, there are things that happen when the baby is with his dad, that are a real problem (for example, if he was allergic to something and they persisted in exposing him to it, or if they were letting him see genuinely inappropriate things on the TV), and it will be far easier for you to object to those if you have built up a good working relationship with his dad (and by extension, with his dad’s family), than if you object to things like this that don’t matter, in the grand scheme of things.
You can’t change what they are doing, in this instance, but you can change how you react to it. Try to find it funny, rather than getting wound up by it. Avoid the pictures on FB if you know they are going to wind you up.
It is so easy for me to say this, and I do realise how much harder it is, for you to do - because this is your precious little baby, and it must be very stressful, having to spend time apart from him.
{{{hugs}}}