I'm not too familiar with WordPress, and modify themes is different for different sites. In general, though, most sites like that have a control panel that you can use to create and modify templates. Failing that, you can make a copy of the template and theme files and modify them in a text editor.
There are several ways to upload files. One is to setup a FTP server on your Linode, and use a regular FTP client like Filezilla. Another possibility is if WordPress has a section to upload files.
Another method is to use scp, which is copy over ssh. If you use Windows at home, you can use WinSCP, which is a GUI tool for using scp in Windows. For Linux, there are different GUI tools to do this; for KDE, Konqueror should work if you go to the location bar and type "sftp://user@yoursite.com" (no quotes, replace user with your user name on your Linode and yoursite.com with the domain name of your Linode, or the IP if you don't have a domain name). SFTP is FTP over ssh, I don't think Konqueror supports regular scp.
If you want to use a Linux or Mac command, use the following command (it's quite similar to the regular cp command):
Code:
scp /path/to/local/file user@yoursite.com:~
Replace /path/to/local/file with the file you want to copy. Replace user with your user name on your Linode. Replace yoursite.com with the domain name OR the IP address to your Linode. The colon ":" is required, along with where you want to copy the file to; the ~ is a shortcut in Linux to your user's home directory. If you have all your images in a single directory, you can add "-r" to the command:
Code:
scp -r /path/to/directory user@yoursite.com:~