Hopefully many of your questions will be answered here: The Photos on Google+ feature is under constant development, I try to keep up with the changes and edit this post accordingly.These screenshots and advice are from a desktop computer running Chrome and a Nexus 5 running Android.
Update on the Community profile photo size:
It used to be 250x250 px but the new required size is 960x540 px minimum. A logo style image cannot be used anymore. The cropped area is really small... At least on a square photo. The bigger the original photo, the better.
Got a good tip from +Dustin W. Stout :
The dimensions I recommend are 1036 x 1036. Bigger is better because it's better to scale down than scale up. Keep all the important stuff in the middle 836px (so leave a padding of 100px on each side).
Google+ Photos tutorial
⦊ About the Photo sizes on Google+
⦊ Need photo help with Picasa? Good desktop photo autobackup tips and instructions
⦊ Make your own GIF's on Android
⦊ Auto Backup
⦊ Be careful when deleting photos from your phone!
⦊ Search from your own Photos tab
⦊ Account photo limits on Google+ and Picasa Web
⦊ Google+ settings for photos
⦊ How to display the large image thumbnail on Google+ link posts
⦊ A tip for Blogger or Blogspot users: Blogger, Google+ and images
⦊ Make a meme on Google+, use it in your blog and other social media, too
⦊ Albums, moving photos and "Photos" tab in Profile view
⦊ Editing an already published image
⦊ See your Drive photos in Google+ (and a few other improvements)
⦊ Having trouble with your profile photo?
⦊ The cover photo on Google+
⦊ About the Lightbox view
⦊ Share a Google+ photo in Facebook
⦊ Instant upload / Auto backup images and privacy
⦊ 3 easy steps to search Auto Backup on the Google+ Photos App
⦊ Google+ Photobox
⦊ Worried about copyright?
⦊ Tip about Photo search