I try not to (sometimes it's too hard to resist with my cat who I find adorable). I am usually against the idea of dressing up animals because they are not conscious of what is going on and certainly did not give their consent. But if it were cold for example, perhaps it is in their best interest!
If it's only now and then for an occasion like Halloween it's harmless, providing the dog is enjoying it too and it's not distressing them in any way. Pets are members of our family and we all like to dress up and have a bit of fun.
I don't have a picture because my dog wasn't particularly cooperative, but he did wear a plastic bat tied to his collar on halloween. Nothing elaborate, but the local kids loved it and all told him how beautiful he looked. Perhaps he won't complain so much about getting dressed up next year now that he knows he gets attention for it!
I have never dressed a pet up. I am not really in favour of it but if the animal didn't seem to mind and it was only once in a while I guess there's no harm. I think my chihuahua would be confused and disturbed if I tried to dress him up. He is 14. Perhaps if I had got him used to the idea when he was young he would be okay with it but it's just not something I have ever wanted to do.