Here are a couple of previous threads about posting pictures.
I can tell you this. If your pictures are too large they will not post. I strongly recommend 800x600 as a good post size.
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Here is a copy of a post that Rich Z did awhile ago... hope it helps.
Probably the easiest thing to do is to just use the photo gallery on this site...
http://www.corvetteflorida.com/photopost/
Click on the link to Upload Photos and fill in the blanks. This will pull the photos directly from your own computer.
Once you do that, there is a section underneath of your photo in the gallery that says "UBBCode:" with a URL to the right of it. Simply click on the button to the right of that URL that says "copy to clipboard", then in any message you post, simply use the control-v function from your keyboard to insert that image into your posts. Or you can use the Paste menu option from your menu bar in the Edit section of your browser.
Pretty easy to do once you try it once or twice.