After you post your thread explaining what you want you will hit "Post"
Once it appears there will be 5 things you can do at the bottom of your post, "Reply" "Edit" "Attach" "Notify" and "Delete".
Click "Attach" it will open up and you can browse your files to upload. Click the picture you want to attach, if you want the picture to appear in the post click "embed" if not it will just be a link on your post we would have to click which would open the pic file. Once you have the picture attached then click "Upload".
If your picture is to large you can open the picture in Windows "Paint" if you have it, you can resize it easily there with the "Resize" option. Usually set mine around 1200 or less. Good size to fit on here without being to big where you can't see the whole pic. Easy peasy...
Hope this helps,
Shawn
