Walms I use Mediafire for my screenshots. It is a free service. Once you've set up an account there you would click "Upload". This will take you to a drop down box giving you an option to choose from computer or the web. Choose computer. This will take you to an "uploader" screen. Click on the center of that and it will take you to your computer where you would browse to find where you have stored your screenies. For me I keep them in a file folder on my desktop so it's easy to find. Open that folder, select the picture or group of pictures you want to upload and select "Begin Upload". This will put them in your Mediafire files. You then click on the picture which will take you to where it shows open. Right click on the picture and select "Copy image URL". Now, in order to post that here, look up at all those items above your "Post Reply" . Under the large B is an icon that says "insert image". Click on that and it will put it in the reply box with a blinking cursor in the middle. Now you hit "paste" from your tool bar or just go with Control v to paste. The url will then show up between the brackets where the blinking cursor was. Next, check "Preview" to make sure the picture actually shows up. If it does you are good to go and can either type more or just hit post.
The steps are similar for Photobucket. I don't use Photobucket because I've lost pictures there and there are very few you can actually keep there.