well I would suggest you have your image processor correctly selected and the proper size options you wish filled out.
You either need to set the proper server path to convert where asked for imagemagick and set your processor to use that or set your processor to GD2
try both see if one work and one doesnt. without you telling me what errors you get when uploading its hard to comment. If the image is not getting resized down and you have the options filled out correctly there should be an error.