From the site FAQ:
First you need to browse to the Gallery. Click on the "Gallery Links" on the menu and scroll down to "Upload Photos".
Uploading a Photo is accomplished thru the use of this form. By filling in the variables, you are telling PhotoPost which photo you wish to upload and information about the photo.
Choose a cateogry - select the most appropriate place for your photo using the drop down list of available categories.
Photo to upload - using the Browser button, select the file on your system you wish to upload (be sure that the file does not exceed size limitations)
You can also upload a ".zip" file containing multiple photos. You can do this by browsing to the .zip file rather than the actual photo as stated above.
Disk space remaining for your account - This tells you how much disk space you have left for uploads
On the next page you will be prompted to enter the following information for each image you uploaded:
Enter a title for the photo - This is a short description of the photo
Keywords - enter up to 10 descriptive terms people can use to search for your image
Photo Description - This area provides for a more detailed description of the photo
Do you want to be notified by email when users post replies? Check this box if you would like an email whenever someone posts a comment to your photo.
Supported image types include: JPG, GIF and PNG.
Support multimedia type (if enabled): MPG, AVI, ASF, WMF, MOV
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