First thing you can do is download infranview:
http://www.irfanview.com/main_download_engl.htm
After it's downloaded, install it to your computer. After it's installed, click on it.... go to "file" at the top and click on "open". Now, find your picture, click on it and click on "open". Now, click on "image" at the top, and scroll down to "resize"... click on it. Now, change the numbers to be 600 x 800 and click "OK". Now, go to "file" at the top, go down to "save as" and save it. Then go to yorkietalk and upload your newly sized picture! Voala! Now, you're a pro

GOOD LUCK!