Your FTP client will have an option for setting the permissions (CHange MODe) on files and folders. It might be in a menu item called "Info" or something like that.
Usually it will show a grid with "owner, group, world" and "read, write, execute".
The folders all will need to have the "execute" permissions set for all three of owner, group, and world, since this means that they can open the folder.
Any folders that a script will write to (such as copying an image file into the folder, as the MaxiGallery script does) will need to be "world-writable", that is, have the Write permissions set for all three owner, group and world as well. And, of course, read permissions, which does exactly as it says, give permission to read a file.
This is an important concept for any webmaster to understand, so go get a cup of (insert favorite beverage here) and do a bit of light reading:
http://www.perlfect.com/articles/chmod.shtml
http://www.onlamp.com/pub/a/php/2003/02/06/php_foundations.html
Unless, of course, you are using a Windows server, which uses a totally different permission system; others can help you with that, I’m not at all familiar with it.