I mean, the issue only happened when he was posting images through oEmbed (using the image URLs), but it was working okay when he actually uploaded images to his BuddyPress installation.
I soon talked to a friend that was using Amazon S3 in his buddypress installation and he said he had the same issue!
A quick search through the code lead me to the CSS class bpfb_images, which is inside of the file images_tag_template of the Activity Plus Plugin.
There surely is a better way to resolve the issue, but this is a quick fix everybody can do.
Picture of the linked image: Size 1318 × 839
What follows is the linked image showing up in BuddyPress after the fix. It was resized to 350(H) X 450(W)
The Fix: Go to buddypress-activity-plus\lib\forms\images_tag_template and add the highlighted code.
Hope it helps. This is not the most elegant solution, but it works.You can even use thickbox for linked images....Don't forget to pay a visit to our spons ;-)