If the photo size is too big you need to resize it.
Use a photo-editing software program (there is a good chance one came with your camera or printer) and resize it.
Some forums and picture posting sites allow only a certain size 800x600 seems to be the norm, some are smaller some don't care. Find what is acceptable and resize it to that.
On this website I use a hosting agent (photobucket.com) it is free and you can upload your pics there, and it gives you the option to (it may be by default now I can't recall) upload it at 800x600.
Upload it there, then copy the direct url it gives you, come to this site, click on
, insert that url ,and voila your pic will be on the thread.