Respect the copyright of others. This means don't steal photographs that other people have taken and pass them off as your own. And yes, Flickr is for photos. With some exceptions, it's OK to post other images, but if the majority of your photostream contains content other than photographs (like illustrations, screenshots, diagrams, etc.) it's very likely that your account will be marked Not in Public Site Areas (NIPSA). NIPSA means your photos won't show up in photo searches, but they will still be visible in your pages, your groups and contacts.