how to attach to the post:
drag&drop to the section "Upload attachment", which is at the bottom of the window where you compose your message:
There are some restrictions on file format, but standard ones (.jpg, .png) are working fine.
how to make an indicator:
1) maybe someone already wrote this. do some search. you can even import MetaTrader indicators into JForex (with more-or less of a success), in case you have found an .mt4 version.
2) code it yourself
3) ask someone to code it for you