I suggest photobucket. It's fairly easy to sign up and use, and easily allows you to upload, resize, and post screenies. I know this, because that's what I use
As for a direct guide:
1. Sign up for a free Photobucket account.
2. Go to the top, where it says "Upload."
3. Hit the big green bar presented before you. This will allow you to navigate through your computer, to locate the files you want to upload. If you have a Windows Computer, starting from the C: drive, you need to click on Program Files (x86), Disney, Disney Online, Pirates Online, and then screenshots.
4. Double click the file to upload it. If you want to download a bunch of files at once, be sure to download the bulk uploader.
5. After it loads, click on the thumbnail of the image.
6. You will be taken to the image. Run your mouse over it, and hit "Resize."
7. Resize the image using the little scrolling things, and hit "Save a Copy."
8. Click on the IMG code in the little box, which usually automatically copies it.
9. Paste it where you want
Just be sure that when you copy over the link from the IMG box over to here, the code is wrapped in [IMG][/IMG], although photobucket automatically adds that to the code.