Playback with your Source and Program Monitors set to Full Rez
Just to point out to others reading this thread....apparently this thread was initially posted to the Adobe Premiere Pro Forum with a Premiere Elements 13 question.
And, under the circumstances, you appear to have offered a Premiere Pro answer to a Premiere Elements question. And, there are differences in this instance.
Apparently you have answered your own question by finding that Timeline rendering was needed to give you the best possible preview of the playback of that Timeline content in the Edit area monitor. If the program is indicating that you need the Timeline content render to get the best possible preview, you render. The preference for "Timeline Render Quality (valid for HD projects)" is not a substitute for Timeline rendering.
Some just in case comments...
1. If your source media properties and the project preset match, then your import will come into the project with no colored line over it. This is the program telling you that you are seeing the best possible preview of those source media at playback in the Edit area monitor. No Timeline rendering is needed nor possible. But, each type you edit that import, then the program will tell you to Timeline render to get the best possible preview and will tell you that, orange line (needs rendering) and green line (rendering complete).
2. You do not have to do this type of rendering. It is a preview matter. But, as you know, it is your window of opportunity to catch a problem sooner than later and is particularly useful when trying to determine what you have when working especially with titles, effects, transitions, and non native formats.
3. A particularly disturbing event is when you save close a project with a rendered Timeline and you re-open it to find it with "unrendered" look (orange line over its content instead of the last seen green line over its content).
1. Is your Timeline exclusively photos or is it a mix of video and photos?
2. No matter how high the resolution of the photos, their resolution in the end product will be determined by the end product format. Just how many
oversized for the project photos you can use will be determined largely by your computer resources.
3. Not sure it will factor into what you are doing, but, if your Timeline content (photos) destination is DVD-VIDEO on DVD disc, the photos appear
sharper in the end product when a 1920 x 1080p DSLR project preset is established before import of the source media.
If you find all of that unacceptable, you have purchased from Adobe direct, and are within 30 days of purchase, you can explore with Adobe details of
return/money back. We are not Adobe here. Rather user to user forum.
We would glad for the opportunity to discuss with you any of the above further.