Your images are probably too large for the site. In order to save space and reduce bandwidth, photographs are limited to 640 x 480 pixels, any uploaded photographs larger than this are resized before being stored on the site. A typical photograph from a 3.1M pixel camera is 2048x1536 pixels and depending upon compression would be around 1MB in size,compared to 160KB for a 640x480 image.
Ideally the image would be automatically resized before uploading, this however raises a number of security issues which most people would not be happy with. The answer is therefore to resize the image before uploading it.
Microsoft Windows XP users can use a 'Power Toys' utility from the Microsoft site HERE more details can be found HERE
Once you have this installed,

1. Open Windows Explorer
2. Open the directory that has the images you wish to upload
3. Select the images you want to upload
4. Right click on the selection and choose "Resize pictures"
5. A new dialogue will be displayed
6. Select "Medium"
7. Click "Ok"

You will now have smaller copies of the original images with "(Medium)" at the end of the file name, use these files when uploading your picture to the site.