Go to Layer > Flatten Image. If prompted to discard hidden layers, click OK.
Go to Image > Image Size and change your resolution to 72 dpi. Change your pixel dimensions to 500x500. (Make sure "Resample Image" is on.)Click OK.
Go to File > SaveAs and choose JPG as the file type. When prompted, choose a compression value of 10. Click OK.
Tips
Each online gallery has its own maximum file size for uploading. Saving your file at the maximum pixel dimensions will allow your layout to be viewed at the largest possible size without any reduction in quality. At DesignerDigitals, this size is 700x700 pixels.
If your file needs to be below a certain size, i.e. under 100K, you can adjust the quality slider in the JPG compression dialog box until you see the file size drop below that value. (The file size is noted under the Preview checkbox.)
If you notice a dramatic change in your layout colors when uploading to the gallery, you may need to change the color profile. In PS, go to Edit > Convert to Profile > select sRGB IEC61966-2.1, with "Perceptual" in the Intent options.
In PSE, go to Edit > Color Settings and click "Allow me to Choose." That will allow you to choose sRGB for web viewing and AdobeRGB for printing.