Make sure you save your picture for the web: it should be 600x600 pixels, and no larger than 150Kb.
If you're using a version of PS, resize your layout to 72 dpi, then 600x600 pixels.
Choose "save for web", make sure it's .jpg format, and move the quality slider until the size (under the preview of the picture) is below 150Kb
Save it somewhere on your hard drive where you'll find it.
Go to our gallery, and click Upload.
Browse to where your picture is on your computer, click ok.
Choose your challenge category
Give it a title
List the credits
Click upload (you'll get an intermediary screen, just click process)
You're good. I think that's all in order, and I hope it's helpful!
Oh, and welcome to MScraps!!