I take it your wanting to post directly from a file in your hard drive verses a 3rd party photo hosting site i.e Fotki,photobucket , etc etc.
The software that Simplemachines (this forum) uses recognizes an URL ....the gate way to an ISP (internet service provider)....the URL to your computer would be your ISP address. An address example would look like this 216.3. 128.12
Some forums will allow you to post a picture (file) directly from your hard drive which is very risky to everyone involved since their software does not require an URL to post pictures.
Here is what an URL looks like when used with a 3rd party photo hosting site (disregard the periods) ....................... URL=https://images45.fotki.com/v1636/photos/0/3714830/13988633/USAForddecalLefthand-vi.jpg[/IMG][/URL]........this is the link to a picture in my Fotki acct.