Thanks for the information.
I will summarise what each service provides:
MessageLabs Image Control
Scans and detects potentially offensive images (e.g. pornographic) attached or embedded within emails and supported Microsoft Office and PDF attachments.
MessageLabs Content Control
Email body, subject, header, supported Microsoft Office and PDF attachments are analysed. Email messages and attachments can be scanned for keywords, phrases, URL lists or alphanumeric formulas (such as credit card numbers). Email that triggers a rule is subjected to a range of actions including block, delete or redirect.
From the links above, I have learnt the Office 365 E1 (Exchange Plan 1) offers the following at no extra cost: MessageLabs Content TLS, MessageLabs Anti-Virus and MessageLabs Anti-Spam.
I am keen to use Office 365 E1 but I need to get a like-for-like comparison to ensure users aren't deprived of anything they are used to.