You need to reduce their size - your average modern digital camera takes pictures in a size suitable for printing a poster. That's way in excess of what's needed for posting pictures online. The best size for displaying pictures here is 640x480 pixels, and you'll need to reduce your pictures to that size, and to below 100kb in order to upload them.
You need a software to do that. The software that came with your camera should do the job perfectly well, and you can just refer to your instruction manual. Or if you prefer, you can download a software for free from the internet to do it. A good one that's easy to use is irfanview - available free from
www.irfanview.com