There have been mentions of software applications that can gang resize your photos, but the easiest way it to take them with fewer pixels to start with.
Your camera should have a menu function where you can do such things as put on the date/time stamp (if your camera has that) or choose the resolution of the picture. Scroll through your options and experiment with various sizes until you find one that is appropriate for your size needs.
The camera I most frequently use has the possibility of 4.1 megapixels, which is pretty small compared to many cameras out there these days. It is primitive enough that it offers me a choice of "Fine" or "Standrd". I am assuming "Fine" is using the full 4.1 megapixel capacity and I just leave it on "Standard".
I also have a Kodak EasyShare that is a 12 Megapixel. The menu will allow me to reduce the picture as low as 1.2 Megapixels or crank it up to the full 12 Megapixels. I keep that camera at about 6 megapixels and only use it when I need higher quality photos (very rare in this business). Even at 6 megapixels I get relatively few photos off a set of batteries while my other camera set on "Standard" will take lots of photos before a battery change is needed.
A smaller picture (fewer pixels) makes a smaller file so you can upload without needing to resize.