try to stick to jpg extensions, some work some don't, i know png doesn't work but not sure about others, so when you find the image look for .jpg at the end of the web address
so search for the image you want on google or wherever, and when you find it make sure you have the image isolated in its own window, if you hit full size image on google it will do it, other sites just right click and hit open image in new window. when you have the window, copy the web address, it should be a simple http://www.example.com/1234.jpg for example
go into a thread and type [IMG] and then right after that, no spaces, paste the web address, ctrl+v or on a mac cmd+v and directly at the end of the image type [/IMG], no spaces, don't forget the slash. if you do that right they should surround the web address and then your picture will show up
example:
here's the web address:
http://www.taomartialarts.com/ski/ski_p_alpine_ht.jpg
put the first img bracket [img]http://www.taomartialarts.com/ski/ski_p_alpine_ht.jpg and then put it on the end