Some of you may be a tad bit computer illiterate, heck maybe even illiterate I dunno, but if you can read this than you probably aren't illiterate (ok maybe it wasn't a really funny joke, sue me). Portions of the website require that images posted not exceed a certain file size (300Kb) and sometimes a certain image size (640x320 pixels I think). In order to reduce both you will need some sort of image manipulation program, there are many free ones for Windows XP, Vista and 7 but I'm not to sure about Mac (but I think it has one built in), some of these can be found online also.
 One such website is Pixlr.com (the actual photo editor can be found here), it is an awesome photo editor that is solely based online and it allows you to upload a photo the web and edit it down then to save it and have it automatically download to your computer. Here is a quick tutorial on how to use Pixlr's Online Image Editing Tool.
 I started off with an image that was 2288x1712 pixels and 1MB in size, this is far to big to be uploaded here so I went to pixlr.com and started the editor and this is what was presented to me:
 I clicked on the "Open Image from Computer" button and I was presented with an open file dialog box, you'll know what to do. I selected my favorite photo of my Roundhead brood stag and uploaded it to pixlr and the went to the toolbar of the editor and selected "Image" than "Image Size" than clicked on it:
 I was than presented with a dialog box to choose the sizes (width and height) in pixels:
 I changed the width to 400 pixels and by leaving the "Constrain proportions" option checked my height was also changed to the correct size to match the width I chose:
 I selected ok and my image was now resized to the size is specified for my image.
 I than went on to save my newly resized image by clicking on "Save":
  I selected to save the file as a JPEG since it has a small file size:
 I than changed the name of my file from the original file to one I knew would represent the photo best and changed the quality of the image to 65 so the file size would be smaller:
 After hitting OK I was giving the choice of where to save my edited photo so I saved it to someplace I'd be able to find it easy (like your desktop or documents folder). After the editing my new file was now 400x299 pixels and 35.5Kb in file size, this will allow me to upload the picture to the size without having it blocked and allowing people with slower internet connections to be able to load the photos faster.
 These steps work with many different photo editors like Paint.net (not a website but a free program). Another option for Windows XP users is the Image Resizer Power Toy which is a small utility that allows you to resize photos to pre-defined sizes and has an option for custom sizes. I hope this helps!