Be considerate of the privacy preferences of the users of your site.
Images from Other Sites
Don't include images from other sites on your pages - that allows the owner of the other site to track the users of your page. Instead, copy the image to your own site and point to it there. If you don't have permission to make a local copy of the image, leave it out or use a different image. See Wikipedia:web bug for more information.
Utilize embedded images and other embedded media whenever and wherever you have both the legal right to do and it will serve your viewers well.
The concern mentioned above seems quite dated. Today both cookies and embedded content are ubiquitous on the web. Some limited use is near essential. Today users are able to control their exposure to privacy risks using the setup of their computers and browsers. Services (Flickr, YouTube, etc.) providing remote media and storage and processing along with embedding have proven very useful options for enhancing web sites. You would likely handicap both your site and your visitors if you categorically avoid them.
Sites should not compromise people's privacy or safety, especially children.
Be sure to be in compliance with this federal law: Children's Online Privacy Protection Act of 1998 (COPPA)
Note however that COPPA applies only to commercial sites and excludes most nonprofit organizations. But it can still serve as a useful set of guidelines for congregation sites.