|You might take a look at irfanview:|
It's a free but powerful viewer and you can use it to reduce picture size:
Also, you ask about changing the picture size on your camera. You can lower the resolution. You might want to experiment with various resolutions and see what file sizes each produces but I would think that most online posting would be happy with something around 1024 x 768.
Reducing the resolution now won't change the pictures you've already taken but ones taken after will have smaller file sizes.