Hello Jonna and welcome to Alamy. Since you're on here I'm assuming you already have an alamy account. There is 2 ways:
1.web based online upload - After logging in, at the top right corner should be a button that says 'my alamy'. Roll your mouse over it and a drop down menu comes up. Click on 'my dashboard'. From there is a blue button that says upload. Click on that and follow the instructions.
2.ftp - With a ftp client you just enter the host name (upload.alamy.com), username/email, and password. After you're logged in, just drag and drop to the appropriate folder.
Your first upload needs to be exactly 3 pictures. After that you can upload as much as you want. The following is a link that probably covers this better than I described -