As a generic point: the email is not secure; in particular, it is not encrypted, not when stored, and (possibly) not when transferred either. If the gods smile upon you today, a given email may be encrypted when it is sent (the SMTP servers involved in the transfer may opportunistically use TLS encryption, and the recipient may download his email through an SSL/TLS-powered protocol like IMAPS). However, it is hard to get guarantees on that subject. Moreover, the email will be stored as is (unprotected) on the mail server's disk, and copies may be kept for some time on the intermediate servers' hard disks.
Therefore, the email contents, including the URL pointing to your extra pictures, cannot be considered "very secret". If the attacker is intent on being able to access these pictures, then he will, regardless of your use of HTTPS or not for the said pictures.
Correspondingly, your question makes any difference only against low-grade attackers, or attackers who are limited to eavesdropping on a specific portion of the network. Against those, HTTPS for the extra pictures may be useful if the pictures have something to hide; that is, the contents of the pictures are confidential, and/or the URL used to retrieve them contains some secret data (anyway, if the URL is guessable, then the picture will not remain secret for long; but the picture could be inherently public while the URL could contain something private like a password-like field).
Applying HTTPS will not harm your security (compared to not applying it), but it may imply a higher load on the server, not because of the encryption overhead (which is small, much smaller than usually considered), but because HTTPS tends to prevent proxies from caching data: if you serve the same picture to one million recipients then you will have to serve it one million times when using HTTPS, whereas with HTTP you could hope for all recipients in the same subnetwork to share the same proxy-cache. As with all things related to performance, this should be measured.