Your Pixelpost installation consists of two parts which must both be backupped to be on the safe side.
When you upload images to pixelpost something clever magic is happening behind the scenes but none of this can happen without a little trick known as chmod. The chmod command is an abbreviation of 'change mode' and is most commonly used on unix-based web servers for defining file system permissions. In laymans terms chmod tells your web server what and who can change what in which files and which folders.
Pixelpost requires that you allow 'owner', 'group' and 'other' to be able to 'read', 'write' and 'execute' on your 'images' and 'thumbnails' folders so that it can create thumbnails for you automatically (put simply, it's all just to make your life easier). The chances are that you created these folders with an ftp program (= FileTransferProgramm) and the chances are that the same program will allow you to set the permissions reqiured on these two folders as well. For more information on using chmod consult your manual or a faq released by the creators of your ftp program.
Find the explanation of the FileTransferProtocol at Wikipedia.
A very good tutorial and explanation of what this is all about is here: CHMOD Tutorial
(This may be different if your prefix was altered)