Quite simply, do not send attachments to the mailing lists.

Never.

If you have graphics you would like to bring to attention of the mailing list, post the graphics on your own web site and put links to those graphics into your post.

No attachments includes inadvertant attachments as well, such as VCF cards, GIFs in your signature, etc. When posting to a mailing list, be sure to use signatures and personalities that do not contain any graphical elements.

Attachments are simply inappropriate for mailing lists. They tend to bog down the list server and are often unwanted by most recipients of the list. In many cases, your attachment may be of a format that is not supported by all members of the mailing list.

Be kind and do not send attachments. It improves the likelihood that your messages and opinions will be read and respected by other members of the mailing list.