You don't need to resize them on Flikr. Just select the smaller one that is resized by them. Here's a sample of what I mean. I always upload no larger than 800x600 although usually 640x480 appx. As you can see from the example it shows that you can choose a size before selecting the link.
If you prefer not to post photos that's fine. Just know that some people prefer to view them here rather than click links to have to see something. Up to you though.