I'm far from an expert but have picked up a few things in the last few weeks of being a member here. Presume you either want to upload it as your signiture or for a post. Certain forums on here, e.g. planted tanks, allow you to browse for photos and add them as an attachment directly from your computer when you start a new topic, others don't. Not sure why. Either way, you need to obviously take your photos, I would suggest in the lowest resolution and compression possible on your camera. Then edit it to the correct size. Around 320 x 240 is good to put in a post I think, any larger and people have to scroll across to see your photos. If for your signiture then no more than 400 x 300.
1. Then go to www.imageshack.us
2. Browse for your edited photo.
3. Upload it.
4. Copy and paste the link in the section called "Hotlink for forums 1" either into your post or into your signiture.
5. Preview to make sure it's there.
6. Voila!
Hope that's helpful as I found it a pain in the backside at first as well.