If a "Download to Picasa" button is disabled, then you will get this issue. The links to do this in the "Action" menu are disable in Chrome beginning from Chrome version 42. The reason is that this feature is utilizing the "NPAPI plugin architecture" and to improve security of Chrome this type of functionality is disabled by default now.
There are some solutions on this issue. If you want to download your own photos, then open "Picasa" and use "File"/ "Import from Google+ Photos/Picasa Web" option. You can also use the download feature in Google+ Photos to download a .zip file of an entire album on plus.google.com. You can also change a Google chrome configuration from Credits to Omry and re-enable it to solve this issue. To do so, Enter chrome://flags/#enable-npapi in your URL bar and click the "Enable" link for the Enable NPAPI configuration option. Click "Relaunch" button located at the bottom of the configuration page. and try again.
Quick Tip to Backup your Digital Photos
If you have treasured photographs, or you take photos of everything you like, you won't want to lose them. So, what is the best way to backup photos on an ongoing basis? In the back of your mind, you know you need to backup all of your photos. But where do you start? Here's how to backup your digital life: PicBackMan is the easiest and simplest way to keep your videos safely backed up in one or more online accounts. Simply download PicBackMan (it's free!), register your account, connect to your online store and tell PicBackMan where your videos are - PicBackMan does the rest, automatically. It bulk uploads all videos and keeps looking for new ones and uploads those too. You don't have to ever touch it.