Making Pixels On a Picture Smaller

Im trying to post a picture of me wakeboarding on wakeworld. But after I upload the picture and try to post it, it says that there are too many pixels and that the maxium is 150 pixels. So I was wondering, how do you make the pixels on the picture smaller?
Thanks

It's saying you have to make the picture smaller (max 150).

What type of computer do you have? MAC or PC?

That's actually referring to file size, not pixels. You'll want to use something to reduce the file size of your image. Here is a simple way to do it from Windows. Find the file with the Windows Explorer and right click on it. Choose "Send To" and "Mail Recipient." Then you'll be prompted as to whether you want to use the original pic or to make it smaller. Choose to make it smaller and click Ok. This will create an email message with your image attached. Right click on the image and save it to your computer. Now upload that pic instead of your original.