1. Is DES 'secure enough'?

Well, that depends. Its probably secure against individuals, but anyone with access to a large computer lab or part of a distributed network would be able to crack it fairly quickly (a week - few months). So, that depends on how bad someone wants it, whether its time sensitive (ie. does the information lose its value in less time than it would take to crack the encryption) and what the attackers resources are.

2. should the fact that I send images primarily over the VPN instead of text or other files change what level of encryption I use?
A known-plaintext attack would be faster than a bruteforce search, but this would require the attacker to have access to an plaintext-ciphertext pair. I imagine that if the attacker knows what the file type is he could probably use any file type headers as known-plaintext.

-Maestr0