The picture by your name is called an avatar. If you want one, you need a picture that is no larger than 64x64 pixels and no larger than 8.8 kb. Once you have a picture that fits those requirements, you can go to the menu bar close to the top of the page and click on "UserCP." Then click on "Edit Avatar," then you may upload your picture.
To reduce the size of photos, the easy way is to simply open the picture in MSPaint, then go to the Image menu at the top, then choose Stretch/Skew. Within the window that pops up, you'll have the option to change some numbers that will alter the size of your picture. Make sure you change them to the SAME number, or your photo will be distorted. My digital camera regularly takes large photos, so I usually reduce them to about 40 percent of original size for posting. If you don't like the size of the picture after you change it, you can always edit --> undo.
I hope that helps