You need to speak with someone in the IT dept at your school. They will instruct you on how to upload images via their WiFi or intranet connection, or if it's even possible (i.e. if they have that set up). If you have a computer and you want to transfer images or other forms of data from your phone to that computer, you still need to get permission from the school. The process is basically use a USB cable. Connect that cable to the computer and to your phone. If the computer system will allow this (it's possible to actually turn off USB ports on an IT level) you can then click on My Computer, then on the icon for your phone. Then go to the folder that contains the data, copy it and paste into a folder on the school's computer. Finding the data will likely be a bit of bear as most people never navigate their phones on that low of a level.
But the main thing to realize is that the computer doesn't belong to you. You do not have the right to place data into the computer without authorization. Sure, it's just harmless pictures, but the head of the school's IT dept. may not agree. So protect yourself, make sure you're doing things the right way and ask your school's IT dept for instructions.