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If you're on Android, and assuming that you just want a "good enough" photo to view on your phone or tablet, there's a great app I use called "Tool for Google Photos, Picasa" at the Play Store here: https://play.google.com/store/apps/details?id=larry.zou.colorfullife
I honestly don't know how it does it, and it takes up relatively little space, but I have tens of thousands of photos offline using this app. Obviously, they're not all full resolution. And, it still relies on the very old API from back when Google used Picasa for photos. So, there's one quirk: photos must be in albums because that's how Picasa worked; there was no photo stream, it was all albums that you uploaded to PicasaWeb. (For me, that's not a problem because I used Picasa even before Google bought it, so all my photos going back to the 90's are in albums, and were automatically converted over to Google Photos albums.)
What this means for current photos, however, is that I make sure EVERY photo in Google Photos gets organized into SOME kind of album... even if it's just like "2017 August." As long as it's in an album, Picasa Tool (as it used to be called, and as the Android icon still reads) can see it. Then, you can choose any albums you want to be downloaded for offline viewing.
I can't say enough good things about this app! It doesn't crash or freeze even when I load it on a new phone and start downloading dozens of albums at a time. It gets ALL my photos offline, and it regularly syncs to fetch any new photos from the albums you've selected for offline use. (If you make a NEW album in Google Photos, it will show up in Picasa Tool, but you'll have to select it for offline viewing to enable the download and sync.) I don't think there's any app that has "just worked" like this one does. It's one of the first apps I install on a new device.
Sorry, though, I don't know of anything similar for iOS if you're on an iPhone. This is Android only.
I do this via a 3rd party Android app called "Tool for Picasa, Google Photos" - https://play.google.com/store/apps/details?id=larry.zou.colorfullife
(I describe how I use it here in Step 7: http://www.rainsberger.ca/2016/03/28/photo-organization-workflow/)
I have no idea how it manages to download (phone quality versions) of ALL my photos, but it does! It's a very reliable app, and I have had zero problems with it in years.
As I wrote in my blog post, however, it can only download for offline viewing photos that are already in albums, which is why I am meticulous about putting all my Google Photos uploads into albums, even if it's just like "2017 March at home" for the sake of being in an album and therefore visable to this app.
Hope that helps! I know it's a huge deal for me to have EVERY photo from Google Photos available on my phone offline.
Try "Tool for Google Photos:
https://play.google.com/store/apps/details?id=larry.zou.colorfullife
Picasa as in the online album by google?
You can upload photos directly from your android to picasa.
There are also apps that supposedly help.
If it's resizing you're after (to attach them to an e-mail directly from your android device), an app like Resize MyPix should help. It can resize a bunch of pictures at the same time.
Please note that I haven't tried the linked apps myself. These were simply among the top results on google.
This app let's you manage and delete photos in picasa.
https://play.google.com/store/apps/details?id=larry.zou.colorfullife