I selected 'Start new topic', filled out the Subject, wrote the body of the message, clicked on 'Additional Options...', then browsed my hard drives Photo Folder to select two images I wanted to attach to the topic. I had previously exported these two Raw files from Lightroom 4 to my hard drive converting them to jpgs in sRGB and pruned each of them to sizes less than 4096 KB (3500 KB to be precise)
How do I post pictures to a new topic?
I think you do it as you are trying to do it, but I suggest you use much, much smaller files. Some people here have slow broadband and some are still using dial-up connections; if you make them download a 3.5Mb photo, they're going to get very irritated. As Mike says, 2000 pixels on the long side is perfectly adequate, and I use 80% on the jpeg quality slider when I export from LR.
If you really must make such large images available, I suggest you post links to them, with warnings about file size.