When posting a message in the forum you can add an inline picture provided it's no larger than 3Mb as follows.
Click Attachments > upload > choose a file from the dialogue box that opens. The locations within the dialogue box can be changed depending on where the picture is that you wish to upload, IE My Pictures, Desktop etc. If you're on an iPad, it's where you have your photos stored. You can also right click on an Internet photo and click 'save image as...' then download it for later uploading here.
Once you've clicked OPEN in the dialogue box, you can then select Thumbnail (recommended) or Full Image.
Alternatively you can use an image hosting service such as Imgur, Photobucket or a host of others and simply post the share code directly into the post.
You can also create an album in the gallery and upload pictures in there.