Click the paper clip or scroll down to where it says manage photos. From there browse your computer and hit upload.
The biggest reason people have trouble uploading from their computers is that the photos are too large. Make sure they are resized to no larger than 800 pixels on the longest side. It reduces the file size AND it makes for easier viewing. The forum can resize larger images, but they often still make things wonky so stick to 800. IF they are still too large you can compress them or reduce the size further. I usually try to keep mine at about 640 pixels or less. Makes it easier on everyone.
Hope this helps!