This tutorial is written specifically for Photoshop Elements 4.0 but it will work the same in other versions.
First you duplicate your original, this is to make sure you will keep the layered version if you have or want to make changes in the future. Go to: File - Duplicate. Name the file as you want, I often say title_web so that I know that is the version for the online galleries.
Now that you have your layout duplicated, you flatten your image. Go to Layer - Flatten image - it might ask you to discard hidden layers, click yes.
Instead of all those seperate layers you only have one layer. The layer styles are all integrated and won't change anymore when resizing the layout. This time you are going to resize your image - in most galleries the size is limited to either 500X500px or 600x600px Go to Image - Resize - Image size
Change the sizes as shown below (or to 500x500 if that is required) and click OK.
All what is left to do is to safe for web.
Go to: File - Save for web
A window will open with two versions of your layout. The first version is your flattened and resized version, the version on the right shows you the web optimized version. Usually I change the settings to "high" , quality to "60". Under your optimized version you will see the file size. In most forums - as I mentioned earlier the size is limited to either 600x600 (or 500x500) as well as a file size from around 100KB. In my example you see I am below 100, so that is fine. In some cases - as you work with more elements, more photos etc your file size will increase and you have to play with the settings and put the Quality slider to 50 instead of 60. Just try and check that your file size stays below 100KB. When the settings are fine, you click OK.