Various forums have different rules for posting photos and their specific sizes..... so simply do not allow uploading of photos directly to the forum. This is because the file size of pictures is large....and if they allowed everyone to upload whatever photo they wanted, their allotment of "room" on the server would quickly be filled up.
All that being said, it's generally best to use what's called a "3rd party hosting" site. What that means is that you upload your photos to one of the Photo hosting sites..... and then in order to get them on the forums you go to the photo hosting site, and copy/past a link into your forum post(s). It sounds much harder then it actually is. What I use is.... http://imgbb.com/ mainly because it's a free service. There are better ones, but the majority of them are "pay"...... meaning that you pay a monthly fee for using them. I'm a cheap bugger, and refuse to pay monthly fees, so I only use the free image hosting sites.