Just simply resize the pixel size of the image.
In Photoshop Elements, it's under image>resize>image size, change the units to pixels if needed, then enter 500 in the box representing the long side of the image. The aspect ratio should be locked and it will automatically put in the correct value in the other box. Should be similar function in full Photoshop but I don't have that.
In Paint Shop Pro 7 (I know, it's old) it's under image>resize and you can directly enter the pixel size. I assume it's similar in newer versions too.
Once done, do any more processing as may be required and save the image. Then you can upload to whatever hosting service you use and link to it from the forum.
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