The Upload button triggers a hidden <input type="file"> element which should trigger your browser’s built in file selection dialog box.
If nothing happens when you click it then I suggest you try another browser.
Uploading Custom Avatar: Nothing happens after upload
After uploading a custom avatar you will still be at the Change your avatar box and your previous (or blank) avatar will still be shown. There is no message that the new avatar has been successfully uploaded and set.
Click Save Changes and then refresh the page and you will see your new avatar.
Upload worked for me on Chrome 32, although the auto-crop seemed to not work correctly. Image was initially cropped just fine, but about 10 minutes later it looked like it was trying to squish a rectangular image into a square box hehe
This is a known issue caused by a DOM security error. Your avatars are being stored in the Windows Azure cloud and for some reason the way that Discourse is doing the real-time update after upload causes this security issue. If the images are generated locally on the web server there is no error so something about the image coming from a different URL. It’s on the todo list to fix but it’s a bit lower priority since it’s just more of an annoyance than a serious problem.
Just a quick tip for Safari users:
Button upload wont work as you noticed (dont even bother with old IE. Button works but upload will be stuck on 0% )
simply go to develop menu and then disable styles
You will see your profile site in kinda wierd way (dont worry)
roll down and you will find your current avatar pic. next to it is button (for me it was almost hidden but it works. trust me)
From here it should upload without problem.
when you are done simply reload the Main forum or and enabel styles again.