You can either do a right mouse click save picture or ask the original poster to email you a copy of the picture.
I'd like to elaborate a little. The advice to right mouse click and select "save picture" works, but it's not the best solution. Usually, when a picture is posted here, it is about 800 pixels across and 600 pixels down. When you print a picture, it's best if you have
at least 200 pixels (with 300 being great) for every inch. That would mean that picture saved by right clicking wouldn't look it's best printed any larger than 4"x3". That's why it is better to get an original copy of the picture.
What I personally like to do, is to post a link with the picture shown here. The link takes you to the website (Smugmug) where I store my pictures. Once you are there, you can download the original sized picture. To do that, just put your mouse over the picture and select "Save Photo" on the little menu that pops up.
Some people put links to their original sized pictures near the picture when they post it. I like to make the picture itself a link. It's easy to do. When you are writing your post, you can put in the picture, then select it and press the insert link button (the little global with the paperclick) and put in the link.
I think you can assume that it is OK to download and use any picture posted on this thread. That's not always the case. Some people value their pictures and prefer to sell them or at least control who they give them to. Legally, they have a copyright on the images and you shouldn't be using them without permission. I've found that just about everyone on the Disboards is pretty amenable to letting others download their pictures with permission, but it's always good to ask first if you find one in another thread. Personally, I just take my pictures for the fun of it, so if you see any of my Disney pictures on any other threads that you want to use, feel free.