I originally wrote this post for ADVRider so some of what you do with the link in the forum is specific to a Xenforo based forum, but the important stuff is the Google part and you should be able to figure out what to do on the BARF side...
Here's the scoop on posting images from Google Photos...
First, the images need to be in an album that has been shared with the "Get Link" option. You will not use the generated link to embed photos as that is just a link to the album gallery, but if you do not do this share action then you end up with the situation where you can see your posted photos but no one else can. I have an album called "web" where I keep all my random publicly posted images.
While viewing the album, click on the picture that you want to embed in a forum post so that you are now viewing the single image.
You have to be viewing the single image. If you try to copy the image URL from the album gallery it will not work. I'm going to use that first pic of the ass end of my 990 Adventure.
Right-click on the image and select "Copy Image Address" or whatever gets you the URL of the image.
This is NOT the same as the URL in the address bar of your browser, so do not copy it from there.
This will yield a very long link with some important information at the end. It looks like this....
Code:
https://lh3.googleusercontent.com/A6i6yDw_7-QmzUpF1b6X1YsYDs7u1GR-pWDG8LfYApwXRh7jL1LmsPaoNS1PmfK70KGyIFlp_cg5RSvWfRA3XIyP2SS_ANkYmh7dFixA5PaJHVM0HIXmp0deweC32N8hhYdlMhF1WD__9Ob6tR1V1v0KC2hyTVfnAWpyrGsqFUy4vBqMYhSUSIewil7HQWccAFP_7UCS6-9n72XisOYJvFqOjIG3kKeg27ZZEJM8Iv8YZo8cJNM_bWiBBO_6Ex1_A4_az66bbBo6AulLeLa5PkJsEQjua3Osw5qOFMNBdSKRnAYHsnmCH9NkYXrfwyHb6UBLWf8XOC8KqxEgL2xaGRH_CR1scR-spjsrk8yuuONuv4bqfnypByF5jM9iRLzRLeQ1GBxH-ujZO3ctKhOjjIpx7C2mZ7jaTlK-YuXf_LroaChYAS9ZhHoxlxsWw6s5vE48OWO8G4Y3SUtAyMRpBwkU14PDiyK_YUUCCUNLYA_MfCzxPEbBQmrBeNXySG16G0s2wgD-VSkBmMJhfI_35d236fh4FGLmEqRi-fNVFFfbdR4YoLqNmriDJSJ6gPEeqG6GyX9psYYGH9SOqsazAFEPw0ZzghskBbyEocymPh3COufkRg=w603-h804-no
See the "=w603-h804" at the end of the URL? That represents the rendered size of the image (603px X 804px) and is going to be important in a minute. Note that the exact width specified in the copied URL is not the original size of the photo, but is directly a result of the size of your browser window when you copied the image address.
So now click the image button in the post editor and paste the URL into the dialog box...
Now what you do is edit the end of the URL so that the width is something appropriate for posting in a forum. I find that 800px or 1024px width works great. As you can see from the info in the "single image" view above the original is 1536px wide, so we will set it to 800px wide and remove the rest so the height can sort itself out proportionally.
Now hit insert and the result will be the image sized to 800px wide.
Finally, you can verify that all is well by viewing your post in an "Incognito" browser window - aka porn mode. This will reveal if the album and its pictures are properly shared so others will see them.
Hope that helps folks. Once you get used to the process you get to where you can fairly quickly flip through a gallery and get all the photos inserted. yeah it's a bit of a pain with the forced "upgrade" from Picasa Web, but it's doable.