Now you're ready to add your watermark, re-size, add any special effects, and save your file.
I won't go into watermarking, but I will point you to some great information that Sara Williams, one of the MFT Presents illustrators, has on her website.
Using Your PNG Watermark in Photoshop Elements
Using Your PNG Watermark in Photoshop 7
I usually re-size my photo before adding special effects and saving. Go to the top menu bar, choose "Image," then "Resize," then "Image Size." I set my longest side to 800.