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Google Photos:
1. No. You can only do this if you delete the picture from the Google Photos app or Google Drive
2. No. To download all the photos back to your phone, you have to do it manually. Consider it as a cloud-based storage using the Google Photos app. Also a hue caveat, if you opted for the free "unlimited" storage, your photos WILL be recompressed significantly REGARDLESS of the size.
3. No integration like that. You will have to do it from Google Photos, export and "share" to instagram

Flickr:
1. No. You can only do this from flickr's app/site. There's not even a desktop syncing client that I know of.
2. No. You are saving things to flickr server. To get photos back to your iPhone, you have to download them manually, or rely on your data connectivity to see your photos from the flickr app.
In the end, any other services can only provide a half-way service. Nothing will truly integrate with Photos app on the Mac.
3. Same with above, you have to pick the photo from flickr first, then "share" to instagram.

Unfortunately there will be no truly integrated 3rd party service on iOS to match iCloud.
 
Dropbox is more versatile. Works across all platforms and is cheap.
Dropbox is great and reliable, but not cheap if one doesn't need 1TB. And the free one is only 2GB. (and people are complaining about how "greedy" Apple is with 5GB free iCloud).

iCloud's 99c a month for 50GB is very good price to be honest.
 
Dropbox is great and reliable, but not cheap if one doesn't need 1TB. And the free one is only 2GB. (and people are complaining about how "greedy" Apple is with 5GB free iCloud).

iCloud's 99c a month for 50GB is very good price to be honest.

Image the more you need with Dropbox the cheaper. My company is on the 1TB plan and it gives everyone in my company access to all of our files while giving me all the personal space for my files. Been using it for years and they have been trouble free.

The $.99 price is good for most users.
 
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