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Can you just attach it to a post? Just hit the "Post Reply" button and above the reply window, hit the little paperclip thingy. The when you get to the "manage attachments" window, hit browse and find it on your hard drive. After that, just double click the image on your hard drive and hit "upload". Should work.
Not sure on that one but the largest I've put on there is 1280x1024. You might try to downsize it to that or a little less and it should work. Not sure what you've got, but I use Microsoft Office Picture Manager on mine. Under the tools section, it's got a tool to resize the picture. I just go in and bring it down to a useable size, save it, and upload it.
You can attach a jpeg up to 1.95 mb, with a maximum of 1280x1024 pixels. When you go to attach a file, you will see the file size limits for all of the different allowed file types.