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You have to resize and check the size against the allowable in the posting area. Generally 620 x 400 or a bit smaller will work. Lately I have had to go down to 550 to 575 to get under 97kb for jpg's.
I use Image Shack ... I can use full resolution on the upload to them, to which they create forum code for thumbnails and a link to Image Shack to view ... works great!