there are two formats photobucket provides that are useful on gaia. URL Link and IMG Code. when posting images on gaia in forums and your signature you use the IMG code
[img] image URL here [/img]
When posting pictures on your profile first go to "edit my profile layout" then several options will pop up including theme, content, etc. click on the option that says picture.
then when when that menu pops up click on either the photobucket (while signed in to your photobucket) or URL tabs.
with the photobucket tab you can put pictures on your profile from a list of all of the pictures in the album you're in on your photobucket. choose the picture you want and hit add
With the URL tab use the URL Link photobucket gave you and paste the URL in the box it gives you then click add.
HOWEVER! i just tried to do that, but i couldn't get to any of the other tabs in the picture options. this is most likely another glitch due to the new layout gaia has. the only thing you can do for now is wait for the glitch to be fixed.