It is ideal to re-size your images and to watermark them before uploading them to the website.
This can be a challenge at first but I have included some links to some tutorials on using software to accomplish this for you. The tutorials are only a few minutes long.
Re-Sizing images
Here is a link to a quick tutorial on using a free program that will automatically re-size your images and it will add a watermark to. Though this is easily done in Photoshop/lightroom, I do recommend this program for two reasons. It is FREE, and it is very, very quick. Here is a link to the site you can download this program.
Uploading to Photographix tutorial
Click here to view a quick tutorial on uploading to the website. There may be a feature or two covered that you might not be familiar with.
As always, feel free to contact me with any questions.
Doug